THE 
FUNDAMENTAL  PRINCIPLES 

OF 

PETROLOGY 


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THE  -FUND AMENTAL 
PRINCIPLES  OF  PETROLOGY- 


BY 
DR.  ERNST  WEINSCHENK 

PROFESSOR   OF   PETROGRAPHY  IN  THE    UNIVERSITY   OF   MUNICH 


AUTHORIZED  TRANSLATION 
(FROM  THE  THIRD  GERMAN  EDITION) 


BY 
ALBERT  JOHANNSEN,  PH.  D. 

ASSOCIATE   PROFESSOR   OF   PETROLOGY   IN  THE   UNIVERSITY   OF   CHICAGO 


WITH  137  FIGURES  AND  6«  PLATES 


FIRST  EL-IT  ION 


McGRAW-HILL  BOOK  COMPANY,  INC, 
239  WEST  39TH  STREET.     NEW  YORK 


LONDON:  HILL  PUBLISHING  CO.,  LTD. 

6  &  8  BOUVERIE  ST.,  E.G. 

1916 


Q  & 


COPYRIGHT,  1916,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


THE.  MAPLE.  PRESS*  YORK.  PA 


Oo  U)e 

of 

Caroline  Austin 

this  translation 
is  ~2><t6lcateo 


357413 


TRANSLATOR'S  PREFACE 

The  need  of  a  short  presentation  of  the  fundamental  principles 
of  petrology  as  applied  to  geology  has  suggested  the  translation 
of  the  first  volume  of  Professor  Weinschenk's  Grundzuge  der 
Gesteinskunde.  Without  thereby  expressing  his  belief  or  dis- 
belief in  the  views  of  the  author,  the  translator  has  tried  to  adhere 
as  closely  as  possible  to  the  sense  as  well  as  to  the  mode  of  ex- 
pression of  the  original.  Only  in  the  case  of  the  Osann  classifica- 
tion has  any  additional  material  been  inserted.  The  rules  for 
calculating  analyses  in  this  system  are  readily  accessible  to  the 
German  student  but  not  to  others,  and  in  view  of  the  growing 
use  made  of  Osann's  triangular  diagrams  by  European  geologists, 
the  addition  was  thought  justifiable.  Owing  to  the  uncertainty 
of  shipments  from  Germany,  it  was  impossible  to  obtain  the  original 
electrotypes  of  the  illustrations,  consequently  the  cuts  in  this 
translation  were  reproduced  from  the  figures  in  the  German 
volume,  and  are  not  quite  so  sharp  as  they  otherwise  would  have 
been. 

The  translator  is  deeply  indebted  and  very  grateful  to  Dr. 
E.  A.  Stephenson  for  critically  reading  this  manuscript  and  for 
various  suggestions.  He  is»  also  indebted  to  Miss  Caroline  A. 
Duror  for  carefully  reading  it  and  for  pointing  out  certain  ambigu- 
ous passages.  The  sad  death  of  this  brilliant  and  promising  stu- 
dent permits  the  writer  to  express  his  appreciation  only  in  the 
dedication  of  this  translation  to  her  memory. 

ALBERT  JOHANNSEN. 

THE  UNIVERSITY  OF  CHICAGO, 
August  14,  1916. 


vn 


CONTENTS 

PAGE 

INTRODUCTION  .........;...... ."  .    .... .    ....  1 

DEFINITIONS  AND  SUBDIVISIONS. -.    .    .    . .    .  -.    .5 

THE  SOLIDIFIED  CRUST  AND  THE  CRYSTALLINE  SCHISTS ;,.....  9 

The  Formation  of  the  Earth's  Crust  .  .  . 9 

Characteristics  of  the  Crystalline  Schists  .  .  .  .^ 11 

VULCANISM  AND  THE  ORIGIN  OF  IGNEOUS  ROCKS  .  ^\ 14 

Older  Theories  of  Vulcanism 14 

Petrographic  Observations  on  Vulcanism  .1 16 

Stiibel's  Theory .-.  .  .  .  .  .  ...  .  .  .  .  .  19 

The  Physical  Character  of  Volcanic  Magmas 21 

The  Outward  Manifestations  of  Vulcanism 24 

The  Geologic  Age  of  Igneous  Rocks 27 

THE  COMPOSITION  OF  IGNEOUS  ROCKS  .  . .  .  30 

Mineralogic  Composition 31 

Distribution  of  the  Elements • 33 

Chemical  Composition  of  Igneous  Rocks 34 

Physico-chemical  Laws  of  the  Magma 36 

Action  of  Mineralizers  .  .  ". 41 

Magmatic  Differentiation  .  ....  .  .  . .'.  .^f .  44 

Complementary  Dikes  ..  ...  .....  <s. ..;...  48 

Petrographic  Provinces * . .  .^ . :  .  .  .  53 

Theories  of  Magmatic  Differentiation 54 

Piezocrystallization 55 

Injection  of  the  Adjacent  Rocks ......  61 

Type-mixing U</  . 66 

Graphical  Representations  of  Chemical  Compositions  ..........  68 

ROCK  WEATHERING  .    ......    .    ....    ^^<   -    .    ^   .........  73 

Weathering  in  General  .  .  .  .  ...  .  ../.  .y.  ...-'  ,_.i  '.„/.....  73 

Physical  Weathering ./ . 74 

Chemical  Weathering ^  ..............  74 

The  Weathering  Solutions .........  79 

The  Weathered  Residues ............  80 

Climatic  Zones  of  Weathering ......;...  82 

Chemical  Weathering  of  Former  Periods ....  /  ..........  83 

Organic  Weathering. (/ .  84 

Rock-sculpture  by  Weathering  .  .  . .•  .  .  .  .  .  .  .  •.  85 

Denudation  .  .  :i  .  .  .  . '  y"  "  '  '•  '  •  ~  •  '  '  •  •  ^ 

THE  XATURE  OF  THE  SEDIMENTS  ......  ^  IS.  ............  94 

Composition  of  the  Sediments ^  .......  95 

Mechanical  Sediments 95 

^Eolian  Deposits i  f  .........  r  ......  97 

Alluvial  Deposits .  ......  .  .  . -.  .  .  . .  .  .  .  99 

Glacial  Deposits  .  .  .  ......  .  .  .  .  .  .  ....  -  .  I  .  .  .  102 

Chemical  Sediments  .  .  .-  .  .  .-  . -.  .  .  .  .  • .'  .  .  .  103 

Organogenic  Sediments 106 

ix 


x  CONTENTS 

PAGE 

Diagenesis Ill 

Recent  and  Fossil  Sediments 114 

CONTACT-METAMOEPHISM      ..' ^. 116 

Agents  of  Contact-metamorphism 116 

Contact-metamorphism  Produced  by  Plutonic  Rocks 119 

Contact-metamorphism  of  Argillites 123 

Contact-metamorphism  of  Carbonate-rocks 127 

Contact-metamorphism  of  Basic  Igneous  Rocks 131 

Serpentinization  of  Feldspar-free  Rocks  by  Contact-metamorphism  .    .    .132 

Piezo-contact-metamorphism 134 

Paragenesis  of  Contact-rocks ,    .    .  136 

Contact-metamorphism  by  Extrusive  Rocks 137 

POST-VOLCANIC  PROCESSES 139 

Post-volcanic  Phenomena 139 

Formation  of  Pegmatite 142 

The  Amygdaloids 144 

Mineral-dikes  and  Ore-veins 146 

Varieties  of  Rock  Alteration 149 

Kaolinization 149 

Saussuritization,  Uralitization,  etc 15.1 

Sericitization - 152 

Serpentinization 152 

Formation  of  Talc 153 

Zeolitization 154 

Other  Alterations 1 54 

Metasomatic  Replacement  of  Carbonate-rocks 155 

REGIONAL  METAMORPHISM 156 

Early  Ideas  Regarding  the  Crystalline  Schists 157 

Younger  Crystalline  Schists . 158 

Variability  of  the  Crystalline  Schists 160 

Gtimbel's  Theory  of  Diagenesis 161 

Theories  of  Regional  Metamorphism 162 

Plutonic  and  Hydrochemical  Metamorphism 164 

Latent  Plasticity  and  Fractureless  Folding 165 

Dynamometamorphism 169 

Facies  of  the  Crystalline  Schists   .    .    . v 173 

Summary 174 

Granite  and  the  Crystalline  Schists 176 

JOINTING  AND  TEXTURES 181 

Appearance  of  Surface  Exposures  of  Various  Rocks 181 

Jointing  and  Parting  in  Rocks 184 

Megascopic  Characters  of  Rocks 190 

Internal  Textures 195 

Internal  Textures  of  Igneous  Rocks 196 

Internal  Textures  of  Contact-rocks  and  Crystalline  Schists.    ......  199 

Internal  Textures  of  Sedimentary  Rocks 201 

Mechanical  Textures .  202 

Inclusions,  Concretions,  and  Secretions 204 

INDEX.  .  207 


THE   FUNDAMENTAL 
PRINCIPLES  OF  PETROLOGY 

INTRODUCTION 

Petrology,  or  the  study  of  rocks,  treats  ^  the  origin,  present 
condition,  and  decay  of  rocks.  It  traces  their  history  through 
every  stage  of  their  existence,  and  contributes  to  our  knowledge 
of  the  processes  by  which  the  earth  has  been  brought  to  its 
present  state.  Thus  defined,  petrology  is  one  of  the  most 
important  fundamentals  of  geology,  yet  it  has  been  greatly 
neglected  by  past  and  present  workers  in  the  science.  A  geologist, 
justly,  is  expected  to  have  a  thorough  training  in  paleontology, 
for  without  it  successful  geologic  work  is  impossible.  But  the 
requirement  of  proper  paleontologic  preparation  has  led  to  a 
one-sided  treatment  of  the  subject,  the  petrologic  side  remaining 
undeveloped. 

No  one  who  is  interested  in  unraveling  the  geology  of  a  region 
will  find  fault  with  a  field  geologist  for  having  the  most  complete 
knowledge  possible  of  things  paleontologic,  but  such  knowledge 
does  not  justify  him  in  neglecting  the  assistance  offered  by 
petrology ;  knowledge  especially  indispensable  to  the  working 
geologist.  Whoever  would  comprehend  all  the  phases  of  geology 
must  give  petrology  the  same  weight  as  paleontology,  and  draw 
upon  each  equally  for  his  results. 

The  causes  which  led  to  the  great  preference  given  to  paleon- 
tology cannot  be  discussed  here  in  detail.  Broadly  speaking,  they 
depend  upon  the  fact  that  when  paleontology  started,  all  pre- 
liminaries for  rapid  growrth  were  found  in  the  high  stage  of  develop- 
ment of  zoology,  and  with  this  for  a  starting  point,  it  needed  but 
to  grow.  Its  methods  of  research,  also,  were  relatively  simple,  and 
did  not  first  require  apparatus  and  instruments  of  many  kinds. 
Thus,  spreading  over  a  prepared  field,  it  captured  the  spirit  of  the 
investigator  by  its  surprising  and  easily  reached  results. 

i 


OF  PETROLOGY 


/  Petrography  did  not  find  such  a  road  before  it.  Step  by  step 
it  had  to  prepare  its  own  path;  it  advanced  slowly,  and  was  con- 
tinually repulsed  by  many  obstacles,  until  finally  the  introduction 
of  the  microscope  raised  it  to  equal  rank  with  the  other  sciences. 
But  with  the  introduction  of  microscopic  methods  a  peculiar 
change  took  place.  While  formerly  none  but  geologists  made  use 
of  |  petrographic-research  methods,  and  these  necessarily  of  the 
simplest  and  most  elementary  kinds,  an  entirely  new  trend  now 
came  about.  Rocks  were  examined  simply  to  determine  their 
mineral  components,  and  no  attention  was  paid  to  their  geologic 
relationships.  Thus  there  arose  a  school,  purely  mineralogic 
and  entirely  independent  of  geology,  whose  viewpoint  extended 
but  little  beyond  the  walls  of  the  laboratory  or  beyond  the  field 
of  the  microscope,  and  only  rarely  did  a  practical  geologist  dare 
to  enter  the  sacred  precincts  of  microscopic  petrography.  The 
unsuspected  wonders  then  first  revealed  in  the  rocks,  completely 
fascinated  those  who  had  overcome  the  many  difficulties  of  the 
preliminary  training  necessary  for  microscopic  petrography;  yet 
on  account  of  their  deficient  geologic  training,  few  petrographers 
could  solve  geologic  problems/  Paleontology,  on  the  other  hand, 
gave  an  abundance  of  important  results. 

The  purely  mineralogic  tendency  did  not  persist  a  great  while, 
for  petrographers  soon  realized  that  geologic  research  was  as  neces- 
sary as  microscopic  observation.  Geologists,  however,  had  become 
accustomed  to  the  idea  that  this  science  was  a  hothouse  production 
which  could  not  bear  transplanting,  and  even  at  the  present  time 
a  large  number  of  geologists  regard  with  suspicion,  if  not  with 
direct  distrust,  the  results  obtained  by  petrographic  research^  and 
this  aversion,  instead  of  diminishing,  increases  more  and  more  as 
petrologists  continue  to  destroy  theories  which  had  been  con- 
sidered fundamentals  of  geology^  The  results  of  modern  petro- 
graphic research,  however,  are  of  such  great  importance  to  geology 
that  the  attempt  on  the  part  of  many  geologists  to  exclude  them 
is  an'injury  to  their  science.7 

At  the  present  time,  petrography  does  not  concern  itself  simply 
with  the  microscopic  examination  of  thin  sections.  Its  horizon 
has  been  broadened,  and  the  greater  as  well  as  the  lesser  features 
of  the  rocks  have  become  the  subject  of  its  study.  The  micro- 
scopic object,  certainly,  even  yet  remains  an  important  working 
tool  of  the  craft,  but  observations  in  the  field  and  the  study  of  the 


INTRODUCTION  3 

geologic  relationships  of  the  rocks  to  each  other  have  become  of 
just  as  great  importance.  While  the  great  museums  formerly 
hesitated  to  allot  even  a  little  dark  corner  to  petrographic  collec- 
tions, at  the  present  time  the  rocks  are  given  a  place  of  equal  rank 
with  other  collections.  Rock  specimens  which  show  the  broad 
geologic  relationships  are  now  valued  most,  and  instead  of  being 
tiresome,  side-by-side  arrangements  of  hand-specimens  of  uniform 
size,  these  collections  have  come  to  rank  among  those  that  are 
most  stimulating. 

Petrography,  as  the  youngest  branch  of  geology,  is  still  far 
from  the  stage  where  practically  everyone  holds  the  same  views; 
a  stage  which  has  been  reached  to  such  a  great  extent,  at  least 
apparently,  by  the  other  branches  of  the  science.  Nevertheless, 
geologists  cannot  afford  to  be  antagonistic  to  the  results  of  its 
investigations,  although  this  is  the  attitude  taken  by  the  authors 
of  some  of  the  most  recent  geologic  text-books. 

It  is  a  fact  not  to  be  overlooked  that  the  student  must  have 
had  a  broad  preliminary  training  to  reach  the  desired  goal  in 
petrography;  a  training  which  generally  takes  more  time  and 
effort  than  he  is  willing  to  give  to  a  subject  which  he  considers 
simply  a  subordinate  aid.  For  this  reason  it  has  been  customary 
for  the  instructor  in  geology  to  present  to  his  students,  as  sufficient 
for  their  purpose,  only  those  facts  which  were  known  before  true 
petrography  existed,  and  then  to  console  himself,  and  excuse  his  own 
superior  attitude  toward  the  whole  science,  by  reflecting  that  the 
numerous  rock- types,  which  over-specializing  petrographers  hav,e 
set  up,  have  neither  geologic  significance  nor  justification;  that  the 
new  names  which  arise  in  such  rapid  succession  have  the  ephemeral 
character  of  day-flies;  and  that  even  among  petrographers  who 
have  made  the  science  their  life-work,  but  little  unanimity  yet 
exists  in  regard  to  the  most  important  questions  affecting  geology/ 
But  what  science  has  become  great  without  having  had  its  truths 
promoted  by  opposition  to  its  accepted  views,  and  in  what  science 
have  the  subdivisions  of  the  classification  and  the  setting  up  of 
new  names  grown  to  such  proportions  as  in  paleontology,  which, 
in  spite  of  this,  is  in  such  high  favor  with  geologists? 

The  attempt  here  made  to  give  a  history  of  the  rocks  from  the 
standpoint  of  modern  petrography,  primarily  for  geologists,  may 
appear  a  somewhat  thankless  task.  The  presentation  of  the  most 
important  results  of  petrographic  research  will  encounter  the 


4  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

objection  from  the  geologic  side  that  their  value  to  geologists  is 
out  of  proportion  to  the  difficulty  with  which  the  beginner  can 
grasp  and  observe  them,  while  on  the  other  hand,  the  same  pre- 
sentation will  be  too  generalized  and  too  limited  for  the  specialist 
in  petrography. 


I.  DEFINITIONS  AND  SUBDIVISIONS 

LITERATURE 

W.   CROSS:  "The   Natural   Classification  of   Igneous  Rocks."     Quart.  Jour.  Geol. 

Soc.  London,  LXVI  (1910),  470. 
F.  FOUQU£  ET  A.  MICHEL-LEVY:  "Mineralogie  micrographique  des  roches  eruptives 

franchises."     Mem.  carte  geol.  France,  1879. 

A.  MARKER:  "Petrology  for  Students."     4th  edition,  Cambridge,  1908. 
F.  H.  HATCH:  "Textbook  of  Petrology."     5th  edition,  London,  1909. 
J.    P.    IDDINGS:  "Igneous  Rocks.     Composition,    Texture,    and  Classification."     2 

vols.,  New  York,  1909  and  1913. 

E.  KALKOWSKY:  "Elemente  der  Lithologie."     Heidelberg,  1886. 
H.  ROSENBUSCH:  "Elemente  der  Gesteinslehre."     3  Aufl.,  Stuttgart,  1910. 
Idem:  " Mikroskopische    Physiographic    der    Mineralien   und    Gesteine."     Bd.    II, 

"Massige  Gesteine,"  4  Aufl.,  Stuttgart,  1907. 
J.  ROTH:  "Allgemeine  und  Chemische  Geologic."     Bd.  II.,  "Petrographie,"  Berlin, 

1887. 
J.  J.  H.  TEALL:  "British  Petrography,  with  Special  Reference  to  the  Igneous  Rocks." 

London,  1888. 
CH.  VELAIN:  "Conferences  de  Petrographie."     Paris,  1889. 

E.  WEIXSCHENK:  "Grundziige  der  Gesteinskunde."     II  Teil.     "Spezielle  Gesteins- 

kunde,"  2  Aufl.,  Freiburg,  1907. 

F.  ZIRKEL:  "Lehrbuch  der  Petrographie."     2  Aufl.,  Leipzig,  1893. 

Rocks  may  be  defined  as  mineral  aggregates  which,  with  more 
or  less  constant  composition,  form  geologically  independent  bodies 
and  an  essential  part  of  the  earth's  crust. 

Nearly  all  rocks  are  aggregates  of  different  minerals,  that  is, 
rocks  are  compound  or  mixed.  In  the  strict  sense  of  the  word, 
simple  rocks,  which  consist  of  but  a  single  mineral,  are  extremely 
rare.  Mineral  combinations,  such  as  ore  deposits,  are  not  classified 
as  rocks  on  account  of  their  usual  subordinate  development  and 
their  less  constant  composition,  although  they  possess  the  former 
property  in  common  with  numerous  dikes.  There  is  no  sharp 
line  of  separation.  Thus  dark  mica  may  not  occur  in  the  apophyses 
from  some  granites,  yet  these  aplites  are  still  regarded  as  rocks. 
The  differentiation  may  have  gone  still  farther;  the  feldspar  also 
may  have  disappeared,  and  the  vein-filling  may  consist  almost 
exclusively  of  compact  quartz.  In  spite  of  the  absolute  identity 
in  the  mode  of  occurrence  of  the  aplite  and  the  quartz,  the  latter 
is  regarded  rather  as  a  mineral  vein  than  a  rock. 

5 


6 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


The  constituents  of  a  rock  may  have  crystallized  in  the  place 
in  which  they  occur.  Such  aggregates  of  authigenic  (Gr.  avdu,  on 
the  spot,  and  7171/0^0:1,  to  be  born)  individuals  are  called  crystalline 
rocks.  On  the  other  hand,  those  rocks  whose  allothigenic  (Gr. 
dXXo0i,  elsewhere)  components  were  derived  from  previously  exist- 
ing rocks,  which  were  shattered  and  destroyed  and  later  deposited 
elsewhere  in  secondary  beds,  are  called  clastic  (Gr.  KXcwros,  broken) 
or  fragmental  rocks. 

Certain  rocks,  namely  the  fluids  and  gases  of  the  hydrosphere  (Gr.  vd  wp,  water, 
<r<paipa,  sphere)  and  atmosphere  (Gr.  dr/ws,  vapor)  which  surround  the  solid  part  of  the 
lithosphere  (Gr.  \idos,  stone)  are  neither  crystalline  nor  clastic.  Even  some  of  the 


FIG.  1. — Massive  granite,  Grimselstrasse.     (Gebr.  Wehrli,  Photo.) 

rocks  occurring  as  solid  aggregates  do  not  belong  to  these  two  great  groups,  since 
they  are  built  up  of  amorphous  constituents  formed  in  situ.  Here  belong  the  glassy 
or  hyaline  (Gr.  flaXos,  glass)  volcanic  rocks,  such  as  obsidian  and  pitchstone,  and  the 
amorphous  porodine  (Gr.  Tropos,  pore,  passage,  diveiv,  whirling)  deposits  from  aqueous 
solutions,  the  so-called  colloidal  gels.  The  latter  are  so  subordinate,  however,  that 
they  would  hardly  be  classed  as  rocks. 

Crystalline  rocks  originated  in  various  ways.  If  a  primary 
crust  solidified  over  the  earth,  it  must  necessarily  have  had  a 
crystalline  character.  Molten  masses,  upon  emerging  from  the 
interior  of  the  earth,  crystallize;  aqueous  solutions  not  infre- 
quently give  crystalline  precipitates;  and  the  action  of  igneous 
masses  on  originally  clastic  formations  may  produce  crystalline 
aggregates.  It  is,  consequently,  not  possible  to  use  crystallinity 
as  a  basis  for  petrographic  classification.  It  has  been  customary 


DEFINITIONS  AND  SUBDIVISIONS  7 

instead,  to  divide  the  rocks  according  to  their  mode  of  origin. 
On  this  basis,  three  groups  are  distinguished. 

(a)  Igneous  (Lat.  ignis,  fire)  or  eruptive  (Lat.  erumpere,  to  break 
forth)  rocks  embrace  those  masses  which  were  forced  upward  in 
a  molten  condition  from  within  the  earth,  and  therefore  also  called 
anogenic  (Gr.  avu,  upward),  or  from  their  usual  external  appear- 
ance, massive  rocks  (Fig.  1).     They  are  authigenic  or  primary,  and 
are  typical  of  the  majority  of  crystalline  rocks. 

(b)  Sedimentary  (Lat.  seder e,  to  settle)  rocks  include  all  rocks 
which  originated  by  precipitation  from  circulating  surface  water, 
and  all  rocks  whose  components,  transported  by  any  means  what- 


FIG.  2. — Stratified  limestone  near  Wiesbaden.     (Prof.  Dr.  Klemm,  Photo.) 

ever,  were  deposited  from  above.  They  are  also  called  catogenic 
(Gr.  /card,  down  from)  or,  from  their  usual  character,  bedded  rocks 
(Fig.  2).  The  sediments  are  secondary  and  consist  essentially  of 
allothigenic  constituents.  They  are  types  of  clastic  or  fragmental 
rocks. 

(c)  The  crystalline  schists,  finally,  whose  characteristic  represen- 
tatives were  supposed  to  belong  to  the  oldest  formation  of  the 
earth,  the  so-called  Archean  (Gr.  apxcuos,  ancient)  or  Azoic  (Gr.  a, 
without,  £wri,  life),  form  a  group  whose  genetic  relationships  are 
in  many  cases  uncertain.  They  are  therefore  also  called  crypto- 
genic  (Gr.  KPVWTOS.  hidden)  rocks.  They  consist,  on  the  whole, 
predominantly  of  authigenic  constituents,  and  are  in  part  pri- 
mary and  in  part  secondary  rocks;  the  latter  are  usually  much 
altered. 


8  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

Only  the  outermost  beds  of  the  earth's  crust  are  known,  the  deepest  openings  being 
less  than  one  two-thousandth  of  the  earth's  radius.  That  which  lies  below  is  entirely 
inaccessible,  and  we  can  only  conjecture  as  to  its  condition. 

The  temperature  of  the  earth  increases  with  depth  in  the  outer,  accessible  parts, 
and  while  the  temperature  gradient  differs  greatly  in  different  places,  it  averages  about 
30°  per  kilometer.  Assuming  a  uniform  increase  from  the  surface  downward,  the 
center  would  have-a  temperature  of  about  200,000°,  the  earth's  radius  being  more  than 
6,000  km.  There  is  no  proof  that  this  increase  in  temperature  is  uniform  to  the 
center  of  the  earth,  since  only  a  small  percentage  of  the  whole  is  known,  but  the 
assumption  that  the  interior  has  a  high  temperature  is  not  unjustifiable. 

We  know,  further,  that  the  specific  gravity  of  the  earth  as  a  whole  is  about  5.5, 
or  about  twice  as  much  as  the  average  density  of  the  accessible  portion  of  the  crust. 
Within  the  interior,  therefore,  there  must  be  present  a  great  abundance  of  very  heavy 
minerals,  most  probably  native  iron,  a  constituent  which  also  forms  an  important  part 
of  meteorites,  themselves  fragments  of  destroyed  worlds. 

Further,  the  pressure  in  the  interior  must  be  enormously  greater  than  at  the 
surface,  whereby  the  molecules,  in  the  highly  heated,  presumably  gaseous  masses, 
must  be  so  closely  packed  that  the  gas  does  not  occupy  so  large  a  volume  as  would 
the  same  material  in  the  solid  state  under  less  pressure.  As  a  necessary  consequence, 
enormously  greater  stresses  must  exist  in  the  interior  than  in  the  crust. 


II.  THE    SOLIDIFIED    CRUST    AND    THE    CRYSTALLINE 

SCHISTS 

LITERATURE 

J.  N.  v.  FUCHS:  "t)ber  die  Theorien  der  Erde."     Munch,  gelehrter  Anzeiger,   1838. 

ARCH.  GEIKIE:  "Textbook  of  Geology."     2d  edition,  London,  1885. 

T.  STERRY  HUNT:  "The  Chemistry  of  the  Primeval  Earth."     Geol.  Mag.,  1868. 

J.  HUTTON:  "Theory  of  the  Earth."     1795. 

I.KANT:  "Allgemeine  Naturgeschichte  und  Theorie  des  Himmels-,"  1755.     Reprinted 

in  Ostwald's  Klassiker  der  exakten  Wissenschaften,  No.  12. 
P.  S.  LAPLACE:  "Exposition  du  systeme  du  monde."     Paris,  1796. 
A.  DE  LAPPARENT:  "Traite  de  geologic."     4th  edition,  Paris,  1900. 
H.  LENK:  "Uber  die  Natur  des  Erdinnern."     Erlangen,  1909. 
CHAS.  LYELL:  "Principles  of  Geology."     12th  edition,  London,  1875. 
R.  MALLET:  "On  Volcanic  Energy."     Phil.  Trans.  Roy.  Soc.  London,  CLXIII  (1873), 

I,  147. 

C.  F.  XAUMANN:  "Lehrbuch  der  Geognosie.     2  Aufl.,  Leipzig,  1858. 
J.  ROTH:  "  Allgemeine  und  Chemische  Geologie."     Bd.  III.     "  Die  Erstarrungskruste 

und  die  Kristallinischen  Schiefer."     Berlin,  1890. 
W.  THOMSON:  "The  Internal  Condition  of  the  Earth,  as  to  Temperature,  Fluidity, 

and  Rigidity."     Trans.  Geol.  Soc.  Glasgow,  VI  (1891). 
F.  TOULA:  "Die  Verschiedenen  Ansichten  iiberdas  Innere  der  Erde."     Wien,  1876. 

The  Formation  of  the  Earth's  Crust. — If  we  wish  to  familiarize 
ourselves  with  the  processes  by  which  the  rocks  were  formed,  we 
must  study  the  earlier  phases  of  the  history  of  our  solar  system 
when  the  earth,  perhaps,  was  in  the  form  of  a  molten  mass  sur- 
rounded by  a  thick  gaseous  mantle.  After  a  long  period  of  time 
the  temperature  of  the  surface  may  have  been  reduced  so  far  that 
a  solid  crust  was  formed  over  the  molten  interior;  a  crust  which 
was  not  scoriaceous  like  the  upper  surface  of  a  lava-stream,  but 
uniformly  crystalline  like  granite,  on  account  of  the  extreme  slow- 
ness of  the  cooling  and  the  enormous  pressure  of  the  heavy 
atmosphere. 

During  this  period,  the  temperature  of  the  earth's  surface  Was 
much  above  the  critical  temperature  of  water,  and  the  present 
ocean,  then  existing  as  vapor,  formed  the  chief  constituent  of  the 
gaseous  envelope.  The  atmospheric  pressure,  for  this  reason  alone, 
had  risen  to  over  two  hundred  atmospheres.  In  addition  there 
were  innumerable  other  substances,  gaseous  at  such  high  tempera- 
tures, and  their  combined  weight  produced  a  pressure  at  the  sur- 

9 


10          FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

\ 

face  such  as  is  reached  only  deep  within  the  earth  at  the  present 
time.  By  these  means  the  molten  masses  became  saturated  with 
many  kinds  of  gases  and  vapors;  the  influence  of  which,  as 
mineralizers,  will  be  described  later. 

Not  only  were  the  outer  parts  of  the  molten  earth  overloaded 
with  water-vapor  and  other  gaseous  substances,  but  they  were 
present  in  excess  even  to  its  innermost  parts.  The  enormous 
stresses  produced  by  these  gases,  heated  mucfy  above  their  critical 
temperatures,  counterbalanced  the  enormous  pressure  of  the  at- 
mosphere. In  this  manner  the  first  coat  of  armor  for  the  young 
earth  was  formed,  not  uniformly  and  continuously,  but  often 
disturbed  by  immense  eruptions,  often  torn  by  molten  masses 
gushing  forth  from  the  deeps.  As  to  how  long  the  alternations 
of  solidification  and  fragmentation  lasted,  who  would  venture 
a  guess!  As  time  passed  the  fluid  center  was  more  and  more 
firmly  imprisoned  by  the  solid  shell,  until  finally  the  temperature 
of  the  outer  portion  of  the  crust  was  reduced  below  the  critical 
temperature  of  water.  The  vapors  then  for  the  first  time  descended 
as  boiling  rain  upon  the  mantle  built  by  volcanic  forces. 

The  water,  which  became  fluid  under  such  high  pressure  at 
365°,  was  exceptionally  active  chemically,  especially  toward  the 
silicates,  and  dissolved  and  re-worked  the  outer  parts  of  the  earth's 
crust.  It  soon  became  saturated  with  all  possible  substances,  in 
part  removed  from  the  atmosphere,  in  part  derived  from  the 
solidified  crust.  Thus  there  came  to  cover  the  earth's  surface, 
as  yet  unwrinkled  and  unfractured,  a  peculiar,  universal  sea, 
purely  chemical  in  its  activity,  differing  widely  in  composition 
from  our  present  ocean,  breaking  on  no  coast,  and  receiving  detrital 
materials  from  no  stream. 

The  stresses  in  the  interior  probably  even  yet  often  rent  the 
weak  crust;  great  volcanic  masses  gushed  forth  and  were  mingled 
with  the  supersaturated  aqueous  solutions  of  the  primeval  sea. 
The  solidified  crust  itself  became  covered  with  rocks  combining 
in  themselves  the  characters  of  precipitates  and  of  volcanic  forma- 
tions. The  temperature  decreased  more  and  more,  and  the  crust 
became  thicker  and  thicker  by  the  continued  cooling  of  the  interior 
and  by  the  addition  of  erupted  masses  and  precipitates  on  the 
surface,  until  finally  the  exterior  of  the  earth  was  not  very  different 
from  what  it  is  at  present.  As  the  temperature  decreased,  the 
water  lost  a  great  part  of  its  chemical  activity.  The  thickening 


THE  SOLIDIFIED  CRUST  11 

crust  permitted  fewer  and  fewer  fractures,  and  instead  of  the 
continual  small  eruptions  of  the  first  stage,  there  were  occasional 
great  eruptions  which  sent  forth  enormous  flows.  Thus  were 
formed  the  first  wrinkles  and  furrows  on  the  surface,  the  first 
elevations  and  depressions;  the  land  was  separated  from  the  sea! 
The  waters  now  began  to  erode  and  level  the  still  but  slightly  ele- 
vated land.  In  place  of  the  chemical  sediments,  probably  chiefly 
silicic,  of  the  earlier  periods,  there  were  now  formed  deposits 
predominantly  mechanical,  made  up  of  fragments  of  older  rocks. 

With  this  change  in  the  character  of  the  sedimentation,  the 
earth's  surface  soon  became  habitable  for  organisms,  and  the 
stage  of  formation  of  the  oldest  clastic  and  fossil-bearing  sediments 
was  reached. 

Characteristics  of  the  Crystalline  Schists. — If  the  geologic 
column  is  traced  downward,  there  will  be  found,  finally,  in  every 
part  of  the  earth,  a  zone  in  which  the  rocks  are  universally  and 
strikingly  different  in  character.  The  fossiliferous  beds,  from  the 
youngest  Cenozoic  to  the  oldest  Paleozoic,  show  no  fundamental 
petrographic  differences  except  that  the  oldest  rocks  usually,  but 
not  always,  are  more  firmly  consolidated  than  the  younger.  Below 
these  sediments,  however,  a  general  crystalline  character  suddenly 
appears,  almost  invariably  accompanied  by  an  absence  of  fossils. 
These  lowest  deposits  have  generally  been  regarded  as  the  oldest 
formation,  and  are  known  as  the  crystalline  schists. 

These  crystalline  schists,  in  certain  localities,  are  uncon- 
formably  overlaid  by  the  later  sediments.  In  other  places  there 
is  no  sharp  boundary  between  the  two,  and  the  crystalline  forma- 
tion passes  by  imperceptible  transitions  into  the  non-crystalline. 
The  different  members  of  this  old  series  of  crystalline  schists  are, 
in  general,  the  same  everywhere,  and  consist  of  gneisses,  mica- 
schists,  and  phyllites. 

Beneath  the  fossiliferous  formations,  then,  the  crystalline 
schists  are  universally  present.  They  possess,  on  the  whole, 
certain  similarities,  such  as  must  necessarily  have  been  produced 
in  rocks  deposited  during  the  earliest  stages  of  the  crust's  develop- 
ment. Everywhere,  upon  approaching  the  sedimentary  and  fos- 
siliferous formations,  there  may  be  recognized  a  gradual  decrease 
in  the  crystalline  character,  such  as  one  would  naturally  expect 
with  the  gradual  diminution  in  chemical  activity  presupposed 
during  these  primeval  times. 


12  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

A  detailed  examination  of  the  crystalline  schists,  however,  will 
show  that  the  relationships  are  not  so  simple  as  they  appeared  at 
first  sight,  and  that  the  above  conclusions  as  to  the  origin  of  the 
crystalline  schists  is  not  justified. 

Unquestionably  a  universal  crystalline  schist  formation,  consisting  of  gneisses, 
mica-schists,  an4  phyllites,  exists  in  all  parts  of  the  earth  beneath  the  fossiliferous  beds. 
But  in  many  places  the  normal,  typically  developed  rocks  of  the  crystalline  schist 
formation  grade  by  unmistakable,  gradual  transitions  into  younger  instead  of  into  the 
oldest  known  sedimentary  formations,  and  widespread  formations  of  exactly  the 
same  petrographic  characters  as  the  crystalline  schists  are  shown  by  their  fossils 
to  be  of  more  recent  geologic  age.  These  formations  have  been  called  the  younger 
crystalline  schists,  in  an  attempt  to  separate  them  from  those  considered  to  be 
truly  Archean. 

These  young,  even  very  young,  crystalline  schists  are  widely  distributed,  and  their 
origin  must  be  ascribed  to  causes  different  from  those  assumed  for  the  older  formations, 
since  the  condition  of  the  earth's  surface  and  of  the  atmosphere  had  long  since  been  so 
fundamentally  altered  that  a  return  to  chemical  sedimentation  cannot  be  considered. 
Further,  isolated  areas  of  crystalline  schists  containing  well-determinable  fossils 
were  already  known  at  the  beginning  of  the  last  century,  and  later  research  has  led 
to  the  discovery  of  many  more.  In  considering  fossil  evidence  of  this  kind,  all  inor- 
ganic forms,  such  as  Eozoon  (cf.  Part  IX),  formerly  incorrectly  thought  to  be  organic, 
must  naturally  be  disregarded.  All  of  the  definitely  determinable  fossils  found 
in  the  crystalline  schists  are  of  organisms  which  are  well  known  elsewhere,  mostly  as 
type  fossils  of  more  recent  formations,  and  none  of  these  fossils  suggests  an  especially 
primitive  form  of  life.  This  is  all  the  more  remarkable  since  the  fossils  actually  present 
are  not  exclusively  the  more  durable  forms,  but  many  of  them,  such  as  graptolites  and 
plant  remains,  are  especially  perishable. 

There  were  sediments,  without  doubt,  much  older  than  the  crystalline  schists, 
although  the  latter  generally  have  been  considered  the  oldest.  In  South  Africa,  for 
example,  a  whole  series  of  unconformable  systems,  with  individual  members  separated 
by  erosion  surfaces  and  transgressions,  underlies  known  Silurian  or  Cambrian  beds. 
While  these  strata  have  preserved  their  purely  clastic  characters,  nowhere  within 
them  are  any  fossils  found  to  serve  as  criteria  for  determining  their  age.  It  is  note- 
worthy that  these  primeval  strata  are  exposed  in  but  few  places  upon  the  earth's 
surface,  and  that,  even  in  South  Africa,  they  are  underlaid  by  gneiss-like  rocks. 

The  fact  that  these  massive,  undoubtedly  pre-Cambrian  sediments  do  not  occur 
elsewhere  between  the  so-called  Archean  crystalline  schist  formation  and  the  overlying 
normal  sediments,  led  to  the  conclusion  that  they  had  been  destroyed  or  thoroughly 
metamorphosed  in  some  way  during  early  Cambrian  times,  perhaps  by  great  fissure 
eruptions.  Since  the  pre-Cambrian  sediments  in  South  Africa  are  entirely  without 
fossils,  the  corresponding  metamorphosed  rocks  naturally  cannot  be  expected  to 
contain  them.  Furthermore,  in  the  few  places  where  lower  or  pre-Cambrian  sedi- 
ments are  fossiliferous,  for  example  in  New  Brunswick,  the  preserved  forms  are  found 
to  have  very  durable  parts. 

The  petrographic  characters  of  the  crystalline  schists  are  still  less  suggestive  of 
primeval  rocks,  for  although  the  different  members  recur  everywhere  as  gneisses, 
mica-schists,  and  phyllites,  moderately  careful  observation  in  different  parts  of  the 
earth  shows  that  the  only  characteristic  common  to  all  is  their  variability.  Not 
only  does  the  habit  change  from  stratum  to  stratum,  but  the  equivalent  formations 
in  different  localities  show  but  little  analogy  petrographically.  Neither  the  gneiss, 


THE  SOLIDIFIED  CRUST  13 

mica-schist,  nor  phyllite  forms  a  single  unit  in  the  petrographic  sense.  Since  it  must 
be  presupposed  that  uniform  conditions  existed  everywhere  on  the  earth's  surface 
during  the  deposition  of  the  primeval  sediments,  the  rocks  should  be  uniform.  As 
a  matter  of  fact,  the  schists  are  characterized  preeminently  by  their  great  lack  of  uni- 
formity. If  the  so-called  Archean  formation  in  various  regions  is  closely  examined 
from  a  petrographic  point  of  view,  these  differences  will  stand  out  sharply.  There  is, 
for  example,  hardly  a  rock-type  in  the  crystalline  central  chain  of  the  Alps  which  has 
its  equivalent  in  the  surrounding  province  of  crystalline  schists,  and  even  in  different 
parts  of  the  central  chain  itself,  the  continual  change  in  the  development,  formation, 
and  composition  of  the  crystalline  schists  is  readily  visible. 

Crystallinity  and  great  geologic  age  do  not  necessarily  belong 
together,  and  the  crystalline  schists  certainly  do  not  occupy  the 
definite  position  in  the  geologic  column  assigned  to  them  by  some 
geologists.  None  of  the  schists  whose  age  is  determinable  belongs 
among  the  oldest  strata  of  the  earth,  and  it  is  not  at  all  probable 
that  many  of  the  remainder  have  this  great  age.  The  crystalline 
character  of  rocks,  in  many  cases,  is  only  a  secondary  property 
developed  by  later  alteration  processes,  namely  by  metamorphism 
(Gr.  juerd,  fj.op(f>r}.  juera:juop(/>6o//a:i,  to  be  transformed). 

When  the  appearance  of  a  rock  is  due  to  later  metamorphism, 
all  conclusions  as  to  the  original  process  of  formation,  drawn  from 
this  obviously  secondary  characteristic,  are  necessarily  incorrect. 
Petrographic  investigation  of  the  crystalline  schists  has  shown  the 
following  fundamental  principle  of  petrology  to  be  correct:  The 
petrographic  characteristics  of  a  rock  have  no  relation  whatever  to  its 
geologic  age. 

The  earliest  sediments  must  have  had  a  uniform  character,  for 
they  were  derived  from  and  deposited  upon  a  universal,  uniform, 
preexisting  crust,  and  while  it  is  probable  that  the  oldest  deposits 
resembled  certain  of  the  crystalline  schists  in  many  ways,  yet  the 
latter  do  not  possess  the  necessary  homogeneity.  From  our 
present  petrographic  knowledge  we  can  consider  it  only  very 
improbable  that  the  true,  primitive  crust  is  accessible  anywhere 
upon  the  earth's  surface. 


III.  VULCANISM  AND  THE  ORIGIN  OF  IGNEOUS  ROCKS 

LITERATURE 

W.  BRANCA:  "Schwabens  125  Vulkanembryonen."     Stuttgart,  1894. 

Idem:  "Vulkane  und  Spalten."     Comptes  Rendes  X  congr.  geol.  internal.,  1906,  985. 

W.  BRANCA  AND  E.  FRASS:  "Das  vulkanische  Ries  bei  Nordlingen."     Abh.  Akad. 

Wiss.,  Berlin,  1901. 

Idem:  "Das  kryptovulkanische  Becken  von  Steinheim."     Ibidem,  1905. 
H.  CREDNER:  "Elemente  der  Geologic,"  II  Aufl.,  1912. 
R.  A.  DALY:  "Abyssal  Igneous  Injection  as  a  Causal  Condition  and  as  an  Effect 

of  Mountain  Building."     Amer.  Jour.  Sd.,  XXII  (1906),  195. 
C.  DOLTER:  "Die  Petrogenesis."     Braunschweig,  1906. 
Idem:  " Physikalisch-chemische  Mineralogie."     Leipzig,  1905. 
ARCH.  GEIKIE:  "The  Ancient  Volcanoes  of  Great  Britain."     London,  1897. 
G.  K.  GILBERT:  "Report  on  the  Geology  of  the  Henry  Mountains."     Washington, 

1877. 

A.  HARKER:  "The  Natural  History  of  Igneous  Rocks."     London,  1909. 
A.  LACROEX:  "La  montagne  Pelee  et  ses  eruptions."     Paris,  1904. 
P.  SCHWAHN:  "  Die  physikalische  Grundlage  der  Stubelschen  Vulkantheorie."     Him- 

mel  und  Erde,  1<UO,  465. 
A.  STUBEL:  "Ein  Wort  iiber  den  Sitz  der  vulkanisehen  Krafte  in  der  Gegenwart." 

Mitteil.  Mus.  Vdlkerkunde,  Leipzig,  1901. 
Idem:  "Riickblick  auf  die  Ausbruchsperiode  des  Mont  Pele  auf  Martinique  1902- 

1903  vom  theoretischen  Gesichtspunkt  aus."     Leipzig,  1904. 

See  also  the  more  recent  papers : 

A.  BRUN:  "Recherches  sur  1'exhalasion  volcanique."     Geneve,  1911. 
A.  L.  DAY  AND  E.  S.  SHEPHERD:  "Water  and  Volcanic  Activity."     Bull.  Geol.  Soc. 

Amer.,  XXIV  (1913),  573-606. 

Older  Theories  of  Vulcanism. — The  Kant-Laplace  theory  of 
the  origin  of  the  solar  system  is  here  assumed  to  be  correct,  since 
under  it  the  phenomena  of  vulcanism  are  most  easily  explained.1 

Among  the  older  theories  of  vulcanism  is  that  of  Werner,  who  supposed  that  the 
subterranean  burning  of  coal  was  the  cause  of  the  igneous  activity.  Other  writers 
explained  it  as  due  to  the  oxidation  of  pyrite,  alkali  metals,  etc.,  or,  in  more  recent 
times,  to  the  hydrolysis  of  carbides,  but  all  of  these  theories  have  been  abandoned. 

From  the  existing  average  temperature  gradient  of  30°  per  kilometer,  it  might  be 
assumed  that  at  a  depth  of  40  to  50  km.  all  rocks  are  in  a  molten  condition.  On  this 
assumption  the  thickness  of  the  solid  crust  of  the  earth  would  be  less  than  1  per  cent, 
of  the  radius,  a  proportion  shown  by  the  solid  line  in  Fig.  3.  In  this  case  the  earth's 
crust  would  be  extremely  thin  and  in  the  first  stages  of  cooling,  and  a  fluid  or  even 
partly  gaseous  interior  would  make  up  the  greater  part  of  the  earth's  mass.  It  has 

1  The  translator  wishes  to  call  attention  to  the  more  probable  method  of  origin 
suggested  by  PROFESSORS  CHAMBERLIN  and  MOULTON.  See  CHAMBERLIN  and 
SALISBURY:  "Geology,"  3  vols.,  New  York,  2d  edition,  1907. 

14 


VULCAN  ISM  AND  IGNEOUS  ROCKS  15 

even  been  asserted  that  the  central  portion  of  the  earth,  in  spite  of  its  enormous  tem- 
perature, is  in  a  solid  state,  since  crystalline  rocks  occupy  a  smaller  volume  than  their 
corresponding  melts.  A  solid  center  is  doubtful,  however,  for  the  critical  temperature 
at  which  most  of  the  substances  in  the  earth's  interior  would  assume  a  fluid  condition 
under  all  circumstances  would  probably  be  reached  at  comparative  shallow  depths. 

Most  volcanoes  which  are  active  or  were  so  during  the  latest 
geologic  periods  occur  near  seacoasts,  and  they  are  especially 
widespread  on  islands.  Further,  extrusive  lavas  give  off  more  or 
less  vapor,  primarily  water-vapor,  accompanied  by  alkali  chlorides, 
sulphates,  etc.,  all  of  them  characteristic  constituents  of  sea 


FIG.  3. —  A  crust  50  km.  in  thickness  as  compared  with  the  diameter  of  the  earth. 

water.  Based  on  these  observations,  a  hypothesis  was  developed 
that  ocean  water  gained  access  to  the  molten  interior  through 
fissures,  and  caused  volcanic  explosions. 

If  the  various  phases  of  this  theory  are  examined  more  closely,  its  untenability, 
both  from  chemical  and  physical  standpoints,  clearly  appears.  While  normal  volcanic 
exhalations  doubtless  have  the  same  chemical  composition  as  the  chief  constituents  of 
sea  water,  this  is  not  a  proof  that  vaporized  sea  water  is  actually  given  off  by  volcanoes 
and  lavas.  It  is  much  more  probable  that  the  sea  water  owes  its  composition  to  dis- 
solved constituents  of  volcanic  emanations  of  former  geologic  periods  and  to  the 
products  leached  from  the  rocks.  This  equally  well  explains  the  similarity  between 
the  compositions  of  the  two. 

Furthermore,  if  the  water  penetrated  to  the  molten  interior  through  fissures  30 
to  40  km.  deep,  it  must  have  rushed  down  with  enormous  rapidity  to  escape  complete 
volatilization  by  the  country-rock  whose  temperature  must  have  been  much  above 
the  critical  temperature  of  water.  It  is  far  more  probable  that  the  segments  of  the 
earth's  crust,  resting  upon  a  fluid  magma,  would  squeeze  the  latter  upward  through 
the  fissures  and,  forcing  the  water  backward,  lead  to  submarine  volcanoes. 

But  should  the  water,  in  spite  of  all  this,  reach  the  molten  interior  by  some  means 
yet  unexplained,  there  would  be  produced,  at  the  point  of  contact  with  the  magma, 
the  so-called  Leidenfrost  phenomenon,  and  a  vapor-film  would  support  the  column  of 
water  above.  Below  the  point  where  the  water  was  in  the  spheroidal  state,  a  cooled 


16  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

crust  would  develop,  so  that  the  generated  water -vapor  would  not  be  able  to  saturate 
the  melt  and  no  eruption  would  be  produced.  The  vapor  itself  would  probably  rise 
through  the  water-column,  heat  it,  and  eventually  cause  boiling  in  the  sea.  A  sudden 
inrush  of  water,  as  here  assumed,  could  only  take  place  in  open  fissures,  but  that  such 
exist  seems  hardly  probable  in  spite  of  the  fact  that  modern  volcanoes  actually  do 
occur  along  great  fracture  lines.  It  seems  more  likely,  from  the  law  of  hydrostatic 
pressure,  that  all  fissures  extending  to  the  molten  magma  would  be  filled  by  it. 

Great  inland  volcanic  eruptions,  such  as  that  on  Martinique,  which  have  very  much 
the  appearance  of  explosions,  are  entirely  incomprehensible  on  such  assumptions. 
It  is  hardly  conceivable,  with  a  presupposed  fracture  reaching  the  surface,  that  the 
magma  should  break  for  itself  a  different  path  through  the  rock  to  spread  desolation 
inland.  And  how  would  it  be  possible  for  all  of  the  erupted  material  to  be  uni- 
formly saturated  with  moisture  if  the  source  was  a  local  contact  of  the  melt  with  water 
in  the  fissure?  A  solid  crust,  more  than  likely,  would  have  formed  between  the  two. 
Besides  failing  to  account  for  these  great  explosions,  this  theory  also  fails  to  give  even 
a  fairly  plausible  reason  for  the  long-continued  activity,  sometimes  increasing  and 
sometimes  decreasing,  of  other  volcanoes.  In  short,  a  careful  consideration  of  existing 
conditions  shows  that  the  sea  water,  as  such,  cannot  be  the  cause  for  volcanic 
activity. 

The  undeniable  fact  that  volcanoes  occur  along  great  fracture 
lines,  even  though  the  actual  vent  may  be  more  or  less  removed 
from  the  fissure,  has  led  to  a  purely  tectonic  explanation  of  volcanic 
activity;  namely,  the  extrusion  of  volcanic  matter  is  ascribed  to 
the  sinking  of  the  solid  crust  in  other  places.  If,  somewhere,  the 
load  resting  upon  a  magma  is  considerably  increased,  fissures  will 
develop,  and  into  these  the  fluid  magma  will  rise,  according  to  the 
law  of  hydrostatic  pressure,  by  the  sinking  of  another  part  of  the 
specifically  heavier  crust. 

It  is  true  that  igneous  rocks  occur  primarily  where  the  earth's 
crust  has  been  broken  by  great  fractures,  and  especially  in  regions 
where  the  crust  was  under  tension,  thus  producing  favorable 
conditions  for  the  rise  of  the  magma  in  the  fissure ;  yet  the  explosive 
force  of  many  eruptions  can  hardly  be  accounted  for  on  this 
purely  mechanical  theory.  The  phenomena  of  vulcanism,  there- 
fore, will  be  considered  in  greater  detail. 

Petrographic  Observations  on  Vulcanism. — The  petrographic 
study  of  igneous  rocks  clearly  shows  that  vulcanism  cannot  be 
due  entirely  to  tectonic  movements,  and  that  the  molten  magma 
itself  must  possess  certain  peculiar  properties  which  aid  in  the 
eruption.  While  as  yet  no  explanation  of  vulcanism  can  be  given 
which  agrees  with  all  physical  laws,  the  accumulation  of  data  is 
a  step  toward  the  solution  of  the  problem. 

Two  sharply  defined  groups  may  be  recognized  among  igneous 
rocks,  especially  among  those  rich  in  silica  and  the  alkalies.  In 


VULCAN  ISM  AND  IGNEOUS  ROCKS  17 

one  the  constituents  are  more  or  less  uniform  in  size.  Such  rocks 
are  called  granular.  They  occur  in  unusually  large  masses,  and 
solidified  between  preexisting  strata  deep  within  the  earth's  crust. 
They  are,  therefore,  also  spoken  of  as  deep-seated,  plutonic,  abyssal 
(Gr.  a(3v<T(Tos,  bottomless),  or  intrusive  (Lat.  in,  in,  trudo,  thrust) 
rocks.  The  other  group  is  distinguished  by  large  crystals  or 
phenocrysts  (Gr.  <palvu,  show,  /cpwraXXos,  crystal),  contrasting  with 
a  fine-grained  groundmass.  These  are  the  porphyritic  (Gr. 
TToppvpeos,  purple,  in  the  translated  sense  variegated)  rocks.  Geo- 
logically it  has  been  found  that  the  latter  rocks  are  mainly 
lava  streams  which  were  poured  out  upon  the  earth's  surface.  In 
consequence,  they  are  also  called  surface,  extrusive,  or  effusive  rocks. 

The  rocks  of  the  first  group  show  a  holocrystalline  development 
under  all  conditions.  The  second,  when  fresh,  in  many  cases  con- 
tain an  amorphous  remnant  or  "basis"  which  did  not  crystallize 
but  solidified  as  glass,  and  in  this  condition  filled  the  interstices 
between  the  previously  crystallized  constituents. 

The  phenocrysts  and  the  groundmasses  of  porphyritic  rocks 
represent  two  stages  in  the  solidification  of  the  magma,  and  are 
generally  sharply  separated  from  each  other.  The  two  generations 
of  minerals  show  characteristic  differences  in  many  cases.  For 
example,  the  hydroxyl-bearing  members  of  the  hornblende  and 
mica  groups  predominate  among  the  colored  constituents  of  the 
first  generation,  while  hydroxyl-free  augite  occurs  in  its  place 
in  the  second.  These  occurrences  point  to  a  physical  difference  in 
the  conditions  of  formation  during  the  two  epochs,  a  difference 
brought  out  more  clearly  by  the  fact  that  in  many  cases  the  mica 
and  hornblende  of  the  first  generation  are  partially  or  entirely 
resorbed  (Lat.  re,  again,  sorbere,  to  suck  in)  during  the  further 
consolidation  of  the  rock;  that  is,  under  later  conditions  they  were 
unstable. 

Resorption  is  generally  more  complete  the  more  perfectly  the  rock  is  crystallized, 
so  that  entirely  unaltered  biotite  crystals  with  sharp  borders  are  not  rare  in  rapidly 
solidified,  glassy  obsidians,  while  in  rhyolite,  which  is  crystalline,  they  occur  only  as 
imperfect  remnants.  Frequently,  too,  great  quantities  of  mica  flakes  or  hornblende 
crystals,  some  of  considerable  size,  are  found  in  the  ashes  and  sand  which  make 
up  a  great  part  of  the  material  of  the  volcanic  tuffs,  while  in  the  lavas  extruded  from 
the  same  vent,  these  minerals  are  almost  entirely  wanting.  From  these  relationships 
the  conclusion  must  be  drawn  that  the  mica  and  hornblende  crystals  had  formed  in 
the  molten  mass  before  it  was  erupted.  Such  phenocrysts,  formed  when  the  molten 
magmas  still  lay  in  the  depths  of  the  earth,  are  called  intratelluric  (Lat.  intra,  within, 
tellus,  the  earth). 
2 


18  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

The  water  content  of  these  minerals  indicates  the  presence  of  water  in  the  melt 
long  before  its  extrusion,  and  the  instability  of  the  minerals  at  a  later  stage  is  explained, 
aside  from  other  factors,  primarily  by  the  escape  of  water  during  the  eruption. 

Although  denied  by  many  observers  even  in  very  recent  times, 
numerous  observations  definitely  prove  that  the  melt  within  the 
earth  is  thoroughly  saturated  with  water-vapor  and  other  gases. 
These  were  not  derived  during  the  eruption  from  the  vadose  (Lat. 
vadosus,  shallow)  waters,  that  is,  from  the  waters  circulating  near  the 
surface,  but,  with  the  other  constituents  of  the  compound  melt 
which  we  call  the  magma  (Gr.  pay  pa,  dough),  were  condensed  and 
dissolved  in  the  melt  during  the  first  stages  of  the  formation  of  the 
earth.  They  are  therefore  called  juvenile  (Lat.  juvenis,  young). 
This  view  is  supported  by  the  character  of  deep-seated  rocks,  which 
doubtless  were  never  in  contact  with  surface  water.  Besides 
containing  fluid  inclusions  in  the  individual  constituents,  they 
have,  to  a  great  extent,  a  mineral  composition  which  indicates 
the  presence  of  water  during  their  crystallization. 

The  chief  constituents  of  silica-rich  igneous  rocks,  such  as  the 
orthoclase,  quartz  and  biotite  of  granite,  have  never  been  crys- 
tallized artificially  by  simply  melting  them  together.  In  the 
attempted  change  from  the  amorphous  to  the  crystalline  state, 
the  viscosity  is  so  great  that  there  is  a  great  tendency  to  under- 
cooling, and  the  melt  solidifies  principally  as  glass.  Water-vapor 
in  the  magma,  however,  reduces  the  viscosity,  and  these  minerals 
have  been  produced  artificially  by  melting  the  proper  com- 
ponents under  high  pressure  nuthe  presence  of  water  and  other 
solvents.  Since  these  gaseous  substances  promote  crystallization 
and  mineral  formation  at  the  temperatures  here  obtaining,  they 
have  been  called  miner alizers  (Fr.  agents  miner  alisateurs) . 

The  chemical  composition  of  pitchstone  proves  the  presence 
of  water  in  the  volcanic  magma.  This  rock  solidified  rapidly 
under  high  pressure,  and  has  a  water  content  of  10  per  cent,  or 
more,  which  can  only  be  primary.  The  great  significance  of  the 
water  content  is  shown  more  plainly  when  recalculated  to  volume 
proportions,  whereby  this  amount  is  found  to  equal  25  per  cent,  of 
the  volume  of  the  rock  under  normal  temperature.  In  a  cubic 
meter  of  a  silica-rich  magma,  therefore,  the  water  present  would 
amount  to  250  liters  after  cooling.  The  exhalations  from  volcanoes 
show  that  gases  of  which  no  traces  remain  in  the  crystallized  rock 
were  present  in  the  magma. 


VULCAN  ISM  AND  IGNEOUS  ROCKS  19 

Another  petrographic  observation  of  importance  here  is  the 
unmistakable  relationship  frequently  shown  by  the  rocks  of  one 
district,  one  petrographic  province,1  which  distinguishes  them 
from  the  rocks  of  another. 

These  relationships  have  led  to  the  conclusion  that  our  present 
volcanic  eruptions  have  no  direct  connections  with  a  molten 
interior,  for  the  superficial  portion  of  the  central  melt,  which  is  the 
only  part  that  need  be  considered  in  regard  to  igneous  activity, 
must  be  considered  a  more  or  less  homogeneous  mass.  On  the 
theory  of  no  connection  between  the  peripheral  magma  basins  and 
the  interior,  the  petrographic  differences  between  successive  erup- 
tions in  a  single  region,  or  between  those  in  different  regions,  are 
more  easily  explained  than  on  any  other  assumption. 

StiibeFs  Theory. — It  may  justly  be  assumed  that  the  first  crust 
of  the  earth  was  weak  and  full  of  fissures.  The  molten  material 
was  forced  through  these  cracks  and,  spreading  out,  formed  a  cover 
above  the  original  crust,  as  shown  in  Fig.  1,  PL  I.  With  continued 
cooling  the  crust  thickened  inwardly  and  became  more  resistant 
to  the  eruptive  forces.  The  number  of  outbursts  decreased  but  the 
intensity  of  the  individual  eruptions  increased,  owing  to  the 
greater  explosive  force  necessary  to  rupture  the  crust.  Enormous 
masses  of  solidified  material  eventually  covered  the  first  crust, 
and  became  the  foundation  of  Stiibers  Panzer decke  (armor  cover- 
ing). The  magma  of  succeeding  eruptions  penetrated  this  cover 
and  formed  within  it  great  chambers  filled  with  molten  magma, 
in  part  fused  from  previously  solidified  material,  in  part  added 
from  below.  These  are  the  so-called  peripheral  magma  basins 
(Ger.  vulkanische  Herde). 

As  time  passed,  the  Panzerdecke  increased  in  thickness,  not 
only  by  volcanic  activity,  but  by  the  gradual  cooling  of  the  earth 
itself.  The  eruptions  gradually  became  less  numerous  but  in- 

1  The  term  petrographical  province  was  introduced  by  JUDD  (Quart.  Jour. 
Geol.  Soc.j  London,  1886,  54)  to  indicate  the  fact  that  rocks  erupted  in  one  region 
present  certain  well-marked  peculiarities  in  mineralogic  composition  and  microscopic 
structure,  serving  to  distinguish  them  from  rocks  belonging  to  the  same  general 
group  which  were  simultaneously  erupted  in  other  regions.  Previous  to  this,  however, 
VOGELSANG  (1872)  had  spoken  of  the  similarity  of  rocks  and  the  possibility  of  group- 
ing them  according  to  " geographischen  Bezirken."  WASHINGTON  (Carnegie  Publica- 
tion No.  57,  1906,  p.  v)  suggested  the  term  comagmatic  region,  as  not  being  limited 
to  petrographic  characters  of  the  rock.  It  is  used  to  express  the  idea  that  magmas  of 
a  certain  area  have  characteristics  in  common,  whether  these  common  characters  are 
due  to  processes  of  magmatic  differentiation  or  to  other  causes.  J. 


20  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

creased  in  violence  to  break  through  the  progressively  stronger 
shell,  until  finally  a  time  arrived  when  the  strength  of  the  crust 
and  the  inside  pressure  were  in  approximate  equilibrium.  There- 
after, on  the  rare  occasions  when  the  crust  was  ruptured,  the 
convulsions  were  enormous,  and  great  masses  of  molten  material 
were  brought  upward  and  produced  extensive  flows,  or  forced 
themselves  into  the  Panzerdecke  itself,  where  they  formed  pe- 
ripheral basins  of  very  great  size.  Eruptions  of  later  geologic 
times  are  very  insignificant  when  compared  with  these  mighty 
flows,  and  Stiibel  aptly  named  this  period  the  Age  of  Catastrophies. 
With  continued  cooling,  most  of  the  former  conduits  were  closed, 
and  a  section  through  the  crust  and  the  overlying  Panzerdecke  at 
this  stage  probably  appeared  as  in  Fig.  2,  PI.  I. 

From  the  beginning  of  the  superficial  cooling  of  the  earth  until 
this  time,  the  volcanic  activity  must  have  been  on  the  increase, 
but  as  more  and  more  of  the  molten  material  in  the  vents  from  the 
interior  solidified,  vulcanism  gradually  decreased  in  amount,  since 
the  increasing  thickness  of  the  crust  prevented  the  opening  of 
fresh  channels.  The  volcanic  eruptions  now  generally  proceeded 
only  from  the  peripheral  basins,  which  had  lost  all  connection  with 
the  molten  interior.  Although  there  doubtless  were  short  periods 
of  increased  activity,  on  the  whole  the  intensity  from  the  Archean 
to  the  present  has  been  diminishing.  Fig.  3,  PL  I,  shows  the 
stage  of  the  earth  at  the  present  time. 

Assuming  Stiibel's  theory  to  be  correct,  the  events  so  far  described  took  place 
before  the  deposition  of  the  oldest  of  the  sediments  with  which  we  are  familiar,  and 
are  prehistoric  even  in  a  geologic  sense.  The  oldest  recognizable  documents  of  the 
geologic  story  clearly  indicate  that  the  Cambrian  and  pre-Cambrian  periods,  usually 
assumed  to  be  the  time  of  the  earliest  sedimentation,  are  actually  but  late  pages  in 
the  story  of  the  development  of  our  earth.  Thick  formations  preceded  the  oldest 
known  sediments  but  they  have  left  us  no  more  than  traces  of  their  former  existence, 
having  been  practically  destroyed  in  the  Age  of  Catastrophies. 

Stiibel's  theory  offers  a  very  convenient  explanation  of  vulcanism,  but  while  its 
last  stages  agree  well  with  the  events  of  historical  geology,  numerous  physical  observa- 
tions are  opposed  to  it.  The  high  temperature  gradient,  primarily,  makes  it  im- 
probable that  there  is  such  an  enormous  Panzerdecke  as  assumed  by  Stiibel.  While 
the  presence  of  hypothetical  peripheral  magma  basins  at  various  moderate  depths 
may  be  the  cause  for  the  rapid  and  very  dissimilar  temperature  gradients  in  different 
parts  of  the  earth,  yet  this  increase  occurs  everywhere,  and  it  would  be  necessary  to 
assume  a  uniform  distribution  of  such  peripheral  basins  at  different,  but  not  too  great, 
depths.  Another  objection  to  the  theory  is  the  fact  that  small  magma  basins,  dis- 
connected from  the  fluid  center  for  whole  geologic  periods,  must  long  ago  have  given 
up  their  heat  to  the  surrounding  rocks  and  have  solidified,  as  did  the  enormous  masses 
of  deep-seated  granitic  rocks  which  erosion  has  disclosed  in  so  many  places.  If  larger 


PLATE  1. 

THE   FORMATION  OF  THE  EARTH'S  CRUST  ACCORD- 
ING TO  STUBEL. 


FIG.  1. — Formation  of  a  solid  crust  K  which  is  everywhere  ruptured  by  extrusions 
of  the  magma.     The  atmosphere  A  forms  a  dense  zone  of  vapor. 


FIG.  2. — The  crust  during  the  Age  of  Catastrophies.  The  volcanic  activity  of 
past  periods  has  formed  a  covering  (Panzerdecke)  over  the  crust  K.  At  this  stage 
there  are  but  few  conduits  connecting  the  interior  with  the  surface.  Through  these, 
however,  massive  extrusions  and  intrusions  take  place. 


FIG.  3. — The  present  condition  of  the  crust.  All  connection  between  the  surface 
and  the  interior  has  been  cut  off.  In  the  Panzerdecke  P  there  still  remain  molten 
remnants  of  former  intrusions.  These  are  connected  with  the  surface  by  channels 
through  the  sediments  <S,  and  produce  our  small  modern  volcanoes. 

(Facing   page  20) 


VULCANISM  AND  IGNEOUS  ROCKS 


21 


molten  masses  were  intruded  so  long  ago  in  the  Panzerdecke,  they  must  have  gradually 
fused  their  way  downward,  during  the  long  subsequent  periods  of  time,  to  form 
protuberances  on  the  central  mass  itself,  and  volcanic  activity  proceeding  from 
such  an  attached  mass  would  be  the  same  as  from  the  fluid  interior  itself.  Thus 
Sttibel's  theory,  while  it  satisfactorily  explains  a  large  number  of  the  geologic  and 
petrographic  phenomena  of  vulcanism,  leaves  many  as  unsolved  riddles. 

The  Physical  Character  of  Volcanic  Magmas. — Several  decades 
ago  the  volcanic  magma  was  considered  to  be  in  itself  inactive,  and 
the  whole  process  of  vulcanism  was  thought  to  be  exclusively  the 
result  of  tectonic  movements.  At  the  present  time,  at  least  in 
many  cases,  volcanic  phenomena  are  regarded  as  due  primarily 
to  the  physical  state  of  the  magma  itself,  for  while  the  majority 
of  volcanoes  undoubtedly  occur  along  fracture  lines  or  fissures, 
such  explosive  eruptions  as  those  which  destroyed  Herculaneum, 
Pompeii,  and  Martinique,  cannot  possibly  be  assigned  to  tectonic 
causes.  Even  though  accompanied  and  followed  by  earthquakes, 


FIG.  4. — Explosion  tubes  in  the  Swabian  Alb.     (After  W.  v.  Branca.) 

these  were  undoubtedly  true  explosions,  which  could  not  have  been 
caused  by  displacements  of  the  earth's  crust.  The  study  of  such 
eruptions  clearly  shows  that  the  volcanic  magma  itself  must  have 
a  tendency  to  explode.  In  addition,  the  folded,  faulted,  and 
shattered  condition  of  the  country-rock  adjacent  to  large  intrusive 
masses,  the  formation  of  contact-breccias,  and  the  injection  of 
igneous  material  into  schists  to  a  distance  of  over  a  kilometer,  all 
indicate  a  considerable  activity  of  the  magma  itself. 

Especially  substantiating  this  view  are  certain  peculiar  explo- 
sion tubes,  called  diatremes1  (Gr.  5ux,  through,  rpijja,  hole).  These 
cylindrical  or  funnel-shaped  tubes  occur,  characteristically,  in 
undisturbed,  horizontal  strata,  and  in  many  cases  penetrate  them 
in  a  sieve-like  manner.  They  are  filled  with  a  heterogeneous 
mixture  of  volcanic  tuffs — brecciated  material  derived  from  the 
country-rock,  and  rock  fragments  of  all  kinds  brought  up  from 

1  Diatremes  from  the  Swabian  Alb  are  well  described  by  BRANCA.     (Fig.  4.) 


22  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

below.  They  can  be  explained  only  by  the  assumption  of  mighty 
explosions  which  broke  through  the  stratified  cover  by  means  of 
compressed  gas,  as  did  that  which  devastated  Martinique  several 
years  ago.  Such  an  explosion  would  tend  to  blow  the  volcanic 
magma  out  in  the  form  of  dust,  instead  of  permitting  it  to  flow 
as  a  sheet. 

Stiibel  considered  this  ability  to  explode  a  physical  property  of 
the  melt,  which  he  assumed  would  increase  in  volume  at  a  certain 
stage  of  its  cooling  until  it  fractured  the  enclosing  shell.  But  a 
repetition  of  expansion  and  contraction  with  continually  decreasing 
temperatures,  such  as  must  be  assumed  to  occur  to  account  for 
repeated  explosions  of  one  and  the  same  volcano,  is  unknown  in 
silicate  melts.  It  is  true  that  water  reaches  its  least  volume  at 
+4°  and  again  expands  below  this  temperature,  but  since  no 
known  substance  has  more  than  one  such  turning  point,  it  is 
improbable  that  such  a  property  is  possessed  by  the  silicate 
melts. 

The  magma  must  be  regarded  as  a  very  complex  solution  with 
a  high  gas  content,  and  it  seems  probable  that  the  cause  of  its 
ability  to  explode  lies  primarily  in  the  gases  which  are  set  free  by 
the  crystallization  of  the  minerals.  The  cooling  of  the  outer  part 
of  the  fluid  earth,  or  of  Stubel's  peripheral  magma  basins,  would 
cause  the  superficial  layers  to  solidify,  and  at  the  same  time  would 
lead  to  the  separation  of  great  quantities  of  gas,  which  were 
dissolved  in  the  magma  but  which  do  not  enter  into  the  constitu- 
tion of  the  crystallized  rock. 

The  significance  of  these  mineralizers  follows  from  the  state- 
ment made  on  page  18.  The  small  amount  occurring  as  fluid  inclu- 
sions in  certain  minerals  of  the  plutonic  rocks  represents  but  a 
small  fraction  of  the  original  quantity,  for  most  of  them  escaped 
by  diffusion  into  the  country-rock  and  saturated  the  overlying 
beds.  A  part,  however,  remained  in  the  still  fluid  magma,  since 
its  ability  to  dissolve  gases  increased  with  decreasing  temperature. 

The  amount  of  gas  separated  from  the  melt  must  be  much 
greater  than  the  amount  conducted  away,  consequently  there  will 
be  a  continual  increase  in  the  stresses,  and  possibly  eventually  an 
explosion. 

From  the  petrographic  appearance  and  geologic  occurrence  of  igneous  rocks, 
certain  data  in  regard  to  the  physical  behavior  of  molten  magmas  may  be  obtained. 
For  example,  aplite,  which  is  a  differentiation  rock  consisting  only  of  quartz  and 


VULCANISM  AND  IGNEOUS  ROCKS  23 

feldspar,  is  derived  from  granite,  and  differs  from  it  only  in  its  higher  content  of  silica 
and  the  alkalies.  It  must,  however,  have  possessed  a  much  greater  degree  of  fluidity 
than  the  granite,  for  in  many  cases  it  extends  in  fine  veinlets  to  great  distances  into 
the  rocks  surrounding  the  granite  massif.  On  the  other  hand,  laboratory  experi- 
ments show  that  silica-rich  melts  are  very  viscous.  This  difference  between  the 
mobility  of  natural  magmas  and  artificial  melts  can  only  be  due  to  the  presence  of 
water-vapor  and  other  mineralizers  in  the  former,  for  silica-rich  melts  which  have  lost 
these  gases,  in  consequence  of  diminution  of  pressure  by  eruption  or  from  other  causes, 
are  very  viscous. 

On  account  of  their  greater  fluidity,  subsilicic  magmas  are  much  less  dependent 
upon  mineralizers.  Silica-poor  igneous  rocks  do  not  occur  in  fine  veins  as  aplite  does, 
but  much  more  commonly  form  broad,  blunt-ended  dikes.  When  basic  magmas 
reach  the  surface,  however,  they  remain  relatively  fluid,  as  may  be  seen  by  the  extra- 
ordinary extent  of  certain  lava  flows. 

Finally,  from  another  difference  in  the  behavior  of  the  two  end- 
members  of  the  igneous  rock  series,  valuable  conclusions  may  be 
drawn  in  regard  to  their  genesis.  It  has  been  observed  that  crys- 
talline aplite  occurs  only  within  other  crystalline  rocks,  such  as 
granite,  rocks  crystallized  by  metamorphism,  etc.  Nowhere  do 
granular  aplites  occur  in  normal,  unmetamorphosed  sediments, 
for  wherever  they  cut  such  rocks  they  assume  the  glassy  form  of 
pitchstones  or  dense  felsites,  the  latter  formed  by  the  recrystalliza- 
tion  of  the  former.  The  basic  end-members  of  the  igneous-rock 
series,  on  the  other  hand,  for  example  the  lamprophyres,  are  wide- 
spread within  sediments  which  not  only  lie  far  distant  from 
any  volcanic  center,  but  which  are  entirely  unmetamorphosed. 
Doelter's  experiments  showed  that  under  normal  conditions  sub- 
silicic  rocks  crystallize  much  more  readily  than  silicic,  doubtless 
on  account  of  their  different  behaviors  at  their  crystallization 
temperatures.  The  points  of  transition  of  pyroxenes,  olivine,  and 
other  basic  minerals  from  the  amorphous  to  the  crystalline  condi- 
tion, lie  very  near  their  melting  points.  Quartz  and  the  alkali- 
rich  aluminium  silicates,  on  the  other  hand,  do  not  crystallize  until 
they  reach  temperatures  considerably  below  then*  melting  points. 
Pure  melts  of  these  minerals,  therefore,  solidify  before  it  is  possible 
for  their  crystallization  to  begin;  that  is,  at  then-  crystallization 
temperatures  they  have  become  too  viscous  to  crystallize,  there- 
fore they  remain  in  an"  amorphous  state.  Magmas  of  this  kind 
can  form  crystalline  aggregates  only  when  their  freezing  points 
are  lowered  to  their  crystallization  points  by  the  presence  of  foreign 
agents;  they  therefore  crystallize  only  where  the  mineralizers 
retained  in  the  magma  through  pressure  cause  a  lowering  of  the 
melting  points,  and  where  slow  cooling  favors  crystallization. 


24  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

A  small,  crystalline  aplite  dike,  therefore,  indicates  by  its  very 
existence  that  it  cooled  very  slowly  and  under  high  pressure.  This 
could  only  have  taken  place  if  the  surrounding  rocks,  at  the  time 
of  the  solidification,  were  deep  within  the  earth  and  still  hot  enough 
to  permit  very  gradual  crystallization. 

The  Outward  Manifestations  of  Vulcanism. — There  are  many 
outward  manifestations  of  vulcanism,  but  we  are  concerned  here 
only  with  the  rocks  which  are  produced  by  it,  and  can  pass  over 
the  question  as  to  how  far  earthquakes  are  directly  related  to  it. 

When  insurmountable  obstacles  lie  in  the  way,  a  part  of  the 
rising  magma  does  not  reach  the  surface  of  the  earth.  Such 
intrusive  or  deep-seated  rocks  solidify  within  the  crust  itself.  They 


Sedimentary  rocks          Igneous  rocks 

FIG.  5. — Laccolith.     The  igneous  rock  has  raised  and  fractured  J)he  sediments. 

force  themselves  into  places  weakened  by  schistosity,  joints, 
unconformities,  etc.,  and  wedge  the  beds  apart.  In  this  manner 
are  formed  intruded  sheets  or  laccoliths  (Gr.  Xd/c/cos;  cistern)  (Fig.  5), 
the  latter  characterized  in  many  cases  by  the  lifting  and  frac- 
turing of  the  overlying  beds.  Other  intrusions  break  across  the 
strata  and  form  stocks  (Fig.  6),  usually  with  rounded  or  elliptical 
cross-sections.  Protected  by  the  overlying  beds,  these  bodies  cool 
slowly  and  uniformly,  and  the  dissolved  gases,  becoming  insoluble 
by  the  crystallization  of  the  silicates,  pass  off  gradually.  The  rocks, 
therefore,  generally  have  a  uniform,  holocrystalline  texture,  and 
rarely  show  different  periods  of  crystallization. 

The  less  massive  fissure-fillings  generally  have  a  flat  form,  and 
are  called  dikes.     A  system  of  dikes  is  shown  in  plan  in  Fig.  7, 


VULCAN  ISM  AND  IGNEOUS  ROCKS 


25 


and  the  corresponding  vertical  section  in  Fig.  8.  The  textures 
of  dikes  vary  as  much  as  do  their  dimensions.  They  range  from 
pure  glass  in  pitchstones  to  a  holocrystalline  development  in 
granitic  rocks;  from  paper-thin  veinlets  of  aplite  to  enormous 
dikes  hundreds  of  meters  wide. 


Sediments  Granite  Detritus 

FIG.  6.— Granite  stock. 


Still  other  magmas  reach  the  surface  of  the  earth,  and  form  the 
effusive  or  extrusive  rocks.  Their  very  variable  characteristics 
depend  directly  upon  their  different  viscosities  and  gaseous 
contents. 


S3         mm 

Schists  Dike  rocks 

FIG.  7. — Ground  plan  of  dikes. 


Schists  Detritus 

FIG.  8. — Cross-section  of  dikes. 


Ordinarily  the  magma,  rich  in  gas,  ruptures  its  overlying  cover  during  some 
great  eruption,  and  enormous  masses  of  finely  divided  material  are  hurled  forth  to 
become  scoriacebus  or  glassy  in  the  course  of  their  flight.  Besides  dust-like  material, 
or  volcanic  ashes,  there  may  occur  considerable  quantities  of  crystals,  previously  separ- 
ated from  the  melt.  Further,  there  may  be  coarser  lava  fragments,  the  so-called 
volcanic  sand  and  lapilli,  fragments  torn  from  the  country-rock,  and  masses  of  plastic 
material,  scoriaceous  in  character  and  twisted  and  contorted  in  their  rapid  flight 
through  the  air.  The  latter  are  known  as  volcanic  bombs.  All  of  these,  falling  around 


26 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


the  mouth  of  the  crater,  tend  to  build  up  a  cone.  The  magma  itself,  rising  upward, 
may  break  through  the  flanks,  or  pour  itself,  stream-like,  over  the  rim.  Flowing  down 
the  declivity  with  great  rapidity,  it  will  spread  itself,  when  of  low  viscosity,  as  an 
extensive  sheet  over  the  surrounding  plain.  Viscous  outpourings,  on  the  other  hand, 
break  into  fragments  during  flow,  and  form  the  so-called  "block-lava."  Alternating 


FIG.  9. — Ideal  section  through  a  strato-volcano. 

eruptions  of  lava  and  of  this  fragmental  material  or  tuff  form  the  strata-volcanoes 
(Fig.  9).  During  very  violent  explosions  fine  dust-like  material  may  be  hurled  to 
considerable  heights  and  be  widely  distributed  by  air  currents.  This  accounts  for  the 
constant  presence  of  volcanic  dust  in  recent,  deep-sea  ooze. 

In  other  cases  the  rising  lava  is  poor  in  gases;  consequently  no  great  explosions  take 


FIG.  10. — A  lava  sheet  with  channels  through 
which  the  material  was  extruded. 


FIG.  11. — A    lava    dome   (Quell- 
kuppe). 


place,  and  little  or  no  tuff  is  formed.  The  molten  material  wells  out  upon  the  surface, 
where,  if  very  fluid,  it  spreads  out  to  form  extensive  sheets  (Fig.  10)  or  long  streams, 
or,  if  very  viscous,  to  form  cones  with  steep  sides  (Quellkuppen,  Fig.  11).  Such  vol- 
canoes are  made  up  of  a  single  flow  and  are  called  homogeneous  volcanoes. 

Finally,  there  are  volcanoes  which  sent  out  no  molten  lava,  or  only  very  small 
amounts  in  the  later  stages  of  their  activity.     These  are  the  explosion-tubes  or 


VULCAN  ISM  AND  IGNEOUS  ROCKS  27 

diatremes  mentioned  on  page  21,  and  their  tuff-filled  channels  are  cut,  in  many  cases, 
by  younger,  minor  dikes.  Such  an  explosive  eruption  may  break  transversely  across 
the  strata  and  blow  out  the  overlying  rocks  like  a  cork,  or,  if  the  explosive  force  was 
insufficient,  press  upward  a  rock-complex  of  rounded  cross-section,  fracturing  the 
surrounding  rocks  and  reducing  them  to  breccia.  In  these  eruptions  neither  lava  nor 
tuff  may  appear,  since  the  rock-cork  barred  the  exit.  Such  activity  has  been  named 
cryptovolcanic.  Fig.  12  is  a  cross-section  through  the  Steinheimer  Basin,  where  the 
Klosterberg,  a  rounded  knob  of  brown  Jurassic  rocks,  was  forced  upward  a  distance 
of  about  150  meters  through  the  gritty,  horizontal,  white  Jura. 

According  to  the  viscosity  and  the  amount  of  gas  present  in  a  magma,  therefore, 
an  eruption  will  be  explosive,  and  only  fine  dust-like  material  will  be  ejected,  or  it 
will  be  quiet  and  the  material  will  be  entirely  molten  when  it  reaches  the  surface. 
The  strato-volcano  is  the  normal  form,  but  while  by  far  the  greater  number  of  recent 
volcanoes  belong  to  this  class,  the  other  forms  are  not  wanting.  At  Kilauea,  Hawaii, 
the  present  extrusives  are  entirely  free  from  tuffs,  while  in  the  explosions  on  Mar- 
tinique there  was  no  compact  effusive  rock. 


FIG.  12. — Cross-section  through  the  Steinheimer  Basin.     (After  W.  v.  Branca  and 

E.  Fraas.) 

The  Geologic  Age  of  Igneous  Rocks. — Extrusive  rocks  are  being 
poured  out  upon  the  surface  of  the  earth  even  at  the  present  time. 
Intrusive  rocks,  on  the  other  hand,  are  known  only  where  erosion 
has  removed  the  overlying  deposits.  There  can  be  no  doubt, 
however,  that  plutonic  rocks  are  even  now  being  formed  deep 
within  the  earth,  especially  in  small  peripheral  basins;  for  although 
volcanic  activity  was  greater  in  certain  geologic  epochs  than  in 
others,  in  all  of  them  both  intrusive  and  extrusive  rocks  can  be 
found. 

From  the  petrographic  viewpoint  there  is  nothing  more  incor- 
rect than  the  oft  repeated  assertion  that  particular  igneous  rock- 
types  belong  to  particular  geologic  formations.  Intrusive  rocks, 
doubtless,  are  found  more  frequently  in  the  older  beds ;  for  example, 
diabase  flows  are  unusually  abundant  in  the  Devonian,  massive 
quartz-porphyry  flows  in  the  Dyas,  and  melaphyres  especially 
characteristic  of  the  Trias  in  America;  but  diabases,  quartz- 
porphyries,  and  melaphyres  were  also  erupted  in  practically  all 
other  geologic  epochs;  and  there  are  true  intrusive  masses,  un- 
doubtedly quite  recent  in  age,  which  in  all  their  properties 
resemble  the  granites,  syenites,  etc.,  of  the  oldest  periods.  While 


28  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

most  granites  occur  in  contact  with  old  formations,  they  are  not, 
eo  ipso,  very  ancient  rocks,  although  they  are  so  regarded  by  many 
geologists  even  at  the  present  time.  Their  almost  invariable 
contact  with  old  rocks  is  easily  explained,  for  since  they  solidified 
deep  within  the  earth,  it  required  long  geologic  periods  for  the 
gradual  removal  of  the  overlying  sediments.  Younger  rocks 
have  generally  suffered  much  less  erosion,  consequently  young 
plutonic  rocks  are  found  primarily  at  high  elevations  where  erosion 
acted  with  unusual  rapidity.  The  classic  example  of  such  young 
plutonic  rocks  is  that  of  the  Andes  where  Tertiary  granites  were 
first  discovered. 

The  unfortunate  hypothesis  that  certain  igneous  rocks  occur 
only  in  certain  geologic  epochs  led  to  the  widely  accepted  division 
of  igneous  rocks  into  pre-Tertiary  and  Tertiary  post-Tertiary 
groups.  This  classification  cannot  be  upheld  under  modern 
petrographic  views. 

Several  decades  ago  certain  investigators  attempted  to  introduce  a  still  more  de- 
tailed geologic  time-classification  into  petrography.  Thus  Gumbel  called  the  Silurian 
diabase  of  the  Fichtelgebirge  proterobase  (Gr.  irporepo  s,  former)  to  distinguish  it  from 
the  Devonian,  and  Paleozoic  picrite,  paldopicrite  (Gr.  iraXeuos,  old),  etc.,  while  certain 
Hungarian  geologists  turned  the  classification  topsyturvy  by  calling  a  Tertiary  granite 
trachyte !  , 

The  classification  of  igneous  rocks  according  to  their  geologic  age  is  impracticable, 
not  only  from  the  difficulty  of  finding  characteristic  distinguishing  marks  for  the 
different  groups,  such  as  are  necessary  in  a  classification,  but  from  the  impossibility 
of  determining  the  ages  of  innumerable  intrusions.  It  is  true  that  the  geologic  age 
of  the  volcanic  activity  is  easily  determinable  where  sheets  and  flows  occur  between 
sedimentaries,  or  where  they  are  accompanied  by  tuffs  in  which  characteristic  fossils 
are  found.  If  the  fossil-bearing  tuffs  are  wanting,  the  accurate  determination  of 
the  geologic  age  may  be  very  difficult,  since  in  many  cases  igneous  rocks  are  intruded 
parallel  to  the  bedding  planes  of  the  sediments.  The  separation  of  these  more  or  less 
extensive  injections  from  lava-flows  may  be  very  difficult,  especially  in  the  case  of 
basic  rocks,  unless  a  scoriaceous  texture  or  injections  into  the  overlying  beds  indicate 
the  mode  of  origin. 

The  difficulty  in  determining  the  age  of  intrusive  masses  is  due  to  the  fact  that  the 
overlying  rocks  may  represent  the  original  roof  over  the  intruded  material  or  sedi- 
ments deposited  after  the  erosion  of  the  original  cover.  Intrusive  masses,  it  is  true, 
generally  metamorphose  the  enclosing  rocks,  the  metamorphism  being  restricted  to 
the  border  zones,  or  they  send  dike-like  apophyses  (Gr.  dTro-^uas,  an  offshoot)  into 
them.  Furthermore,  the  border  zone  of  the  igneous  mass  itself  may  show  the  effects 
of  contact  metamorphism.  In  beds  subsequently  deposited,  apophyses  and  border 
alterations  are  wanting,  but  in  their  place  the  lowest  member  may  consist  of  a  charac- 
teristic basal  conglomerate  formed  by  the  destruction  of  the  underlying  rock,  and  con- 
taining rounded  or  angular  fragments  of  it. 

The  characteristics  of  the  two  types  are  different  enough  to  make  their  separation 
seem  possible  in  every  case  by  moderately  careful  observation.  Though  apophyses 
be  wanting,  and  the  mineralogic  alterations  within  the  border  zone  of  the  intruded 


VULCAN  ISM  AND  IGNEOUS  ROCKS  29 

mass  hardly  noticeable,  yet  contact  metamorphism  in  the  overlying  rocks  would 
definitely  prove  them,  to  be  the  original  cover:  It  has  become  the  custom,  however, 
to  ascribe  a  greater  metamorphic  power  to  tectonic  pressures  than  to  volcanic  proc- 
esses. Thus,  in  greatly  folded  regions  such  as  the  Alps,  all  of  the  alteration  is  as- 
cribed to  the  mountain-building  forces,  which  makes  it  necessary  to  construct  great 
artificial  faults  to  veil  the  actual  intrusive  form  of  the  gneiss-like  granites.  Elsewhere, 
however,  considerable  faulting  occurs  at  the  contact  of  such  rocks.  This  tends  to 
produce  friction-breccias  whose  included  rock  fragments,  rounded  by  the  faulting, 
so  much  resemble  the  pebbles  in  basal  conglomerates  that  the  two  rocks  may  be 
easily  confused. 


IV.  THE  COMPOSITION  OF  IGNEOUS  ROCKS 

LITERATURE 

H.  BACKSTROM:  "  Causes  of  Magmatic  Differentiation."     Jour.  Geol,  I  (1893),  773. 
W.  C.  BROGGER:  "Die  Mineralien  der  Pegmatitgange  der  siidnorwegischen  Augit- 

und  Nephelinsyenite."     Zeitschr.  f.  Kryst.,  XVI  (1890). 
Idem:  "The  Basic  Eruptive  Rocks  of  Gran."     Quart.  Jour.  Geol.   Soc.,  London,   I 

(1894),  15. 
Idem:  "Die   Eruptivgesteine   des   Kristianiagebietes."     Vidensk.   selsk.   skr.,    1894, 

Nr.  4;  1895,  Nr.  7;  1897,  Nr.  6. 
H.   BUCKING:  "  Mitteilungen  iiber  die  Eruptivgesteine  der  Sektion  Schmalkalden. " 

Jahrb.  preuss.  geol.  Landesanst.,  1897,  119. 
W.  BUNSEN:  "tlber  die  Prozesse  der  vulkanischen  Gesteinsbildungen."     Pogg.  Ann., 

LXXXIII  (1851),  197. 
W.  CROSS:  "The  Natural  Classification  of  Igneous  Rocks."     Quart.  Jour.  Geol.  Soc., 

London,  LXVI  (1910),  470. 

CROSS  ET  AL:  "Quantitative  Classification  of  Igneous  Rocks."  Chicago,  1903. 
A.  DAUBREE:  "Etudes  synthetiques  de  geologic  experimentale."  Paris,  1879. 
C.  DOLTER:  "Die  Silikatschemelzen."  Sitzb.  Akad.  W iss.  Wien,  CXIII  (1904),  Abt. 

I,  177;  CXIV  (1905),  I,  529. 
F.  FOUQUE:  "Recherches  mineralogiques  et  geologiques  sur  les  laves  des  dykes  de 

Thera."     Mem.  pres.  a  I  Acad.,  XXVI  (1876),  Nr.  4. 

F.  FOUQUE  ET  A.  MICHEL-LEVY:  "Synthese  des  mineraux."     Paris,  1882. 
Idem:  "Reproduction  artificielle  d'un  trachyte  micace"."   Compt.   Rendus,   CXIII 

(1891),  283. 
C.  W.  C.  FUCHS:  "Die  Veranderungen  in  der  fliissigen  und  erstarrenden  Lava." 

Tscherm.  min.  petr.  Mitteil,  1871,  65. 
A.  HAGUE  AND  J.  P.  IDDINGS:  "On  the  Development  of  Crystallization  in  the  Igneous 

Rocks  of  Washoe."     Bull.  U.  S.  G.  S.,  No.  17,  1885. 
J.  P.  IDDINGS:  "The  Origin  of  Igneous  Rocks."     Bull.  Phil.  Soc.,  Washington,  XII 

(1892),  89. 
C.  v.   JOHN:  "Uber  die   Eruptivgesteine  von  Jablonica  an  der  Narenta."     Jahrb. 

Geol.  Reichsanst,  XXXVIII  (1888),  343. 
J.  W.  JUDD:  "On  the  Gabbros,  Dolerites  and  Basalts  of  Tertiary  Age  in  Scotland  and 

Ireland."     Quart.  Jour.  Geol.  Soc.,  London,  XLII  (1886),  54. 
A.  LAGORIO:  "tlber  die  Natur  der  Glasbasis  sowie  der  Kristallisationsvorgange  im 

eruptiven  Magma."     Tscherm.  min.  petr.  Mitteil,  VIII  (1887),  421. 
H.  O.  LANG:  "Versuch  einer  Ordnung  der  Eruptivegesteine  nach  ihrem  chemischen 

Bestand."     Ibidem,  XII  (1891),  199. 
A.  DE  LAPPARENT:  "Note  sur  le  role  des  agents  mineralisateurs  dans  la  formation 

des  roches  eruptives."     Bull.  Soc.  Geol.  France,  XVII,  (1889)  (3),  282. 
F.  LOWINSON-LESSING  :  "Studien   iiber  die   Eruptivgesteine."     Compt.  Rendus  VII 

congr.  geol.  intern.,  1897,  St.  Petersburg,  1899. 
K.  A.  LOSSEN:  "Die  Bodegang  im  Harz,    eine  Granit-Apophyse   mit  vorwiegend 

porphyrischer  Ausbildung."     Zeitschr.  deutsch.  geol.  Ges.,  XXVI  (1874),  856. 
A.  MICHEL-LEVY:  "Structure  et  classification  des  roches  eruptives."     Paris,  1889. 

30 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  31 

Idem:  "Sur  quelques  particularites  de  gisement  du  porphyr  bleu  de    1'Est^rel." 

Bull  Soc.  Geol.  France,  XXIV  (1896)  (3),  47,  123. 
L.  MILCH:  "  Die  Systematik  der  Eruptivgesteine.     I  Tl."     Fortschr.  Mineral.  Kristall. 

Petrogr.,  Ill  (1913),  141. 
F.  v.   RICHTHOFEN:  "Uber  den  Ursprung  der  vulkanischen  Gesteine."     Zeitschr. 

deutsch.  geol.  Ges.,  XXI  (1869),  1. 
H.  ROSENBUSCH:  "t)ber  die  chemischen  Beziehungen  der  Eruptivgesteine."     Tscherm, 

min.  petr.  Mitteil,  XI  (1889),  144. 
J.  ROTH:-  "Die  Einteilung  und  die  chemische  Beschaffenheit  der  Eruptivgesteine." 

Zeitschr.  deutsch.  geol.  Ges.,  XLIII  (1891),  1. 
Idem:  "Beitrage  zur  Petrographie  der  plutonischen  Gesteine."     Abhandl.   preuss. 

Akad.  Wiss.,  1869,  67;  1873,  89;  1879,  1;  1884,  1. 
TH.  SCHEERER:  "tTber  die  chemische  Konstitution  der  Plutonite."     Festschr.   zum 

100  jahrigen  Bestehen  der  Bergakademie  Freiberg,  1866,  158. 
A.  STELZNER:  "  Beitrage  zur  Geologic  und  Palaontologie  der  argentinischen  Republik." 

Kassel  und  Berlin,  1885. 
J.  H.  L.  VOGT:  "Beitrag  zur  Kenntnis  der  Gesetze  der  Mineralbildung  in  Schmelz- 

massen  und  in  neovulkanischen  Ergussgesteinen."     Arch.  math,  naturvid.,  XIII 

(1890);  XIV  (1891). 
Idem:  "Die  Theorie  der  Silikatschmelzlosungen."     Ber.  V  intern.  Kongr.  angew. 

Chemie,  Berlin,  1903  (1904). 
Idem:    " Physikalisch-chemische     Gesetze    der    Kristallizationsfolge  '  in     Eruptiv- 

gesteinen."     Tscherm.  min.  petr.  Mitteil.,  XXIV,  (1906),  437. 
Idem:  "Uber  anchimonomineralische  und  anchieutektische  Eruptivgesteine."     Ges. 

Wiss.,  Kristiania,  1908,  Nr.  10. 
S.  v.  WALTERSHAUSEN:  "Uber  die  vulkanischen  Gesteine  in  Sizilien."     Gottingen, 

1853. 
H.  S.  WASHINGTON:  "The  Distribution  of  the  Elements  in  Igneous  Rocks."     Bull. 

Amer.  Inst.  Min.  Eng.,  1908,  809. 
E.  WEINSCHENK:  "  Zur  Kenntnis  der  Entstehung  der  Gesteine  und  Minerallagerstatte 

der  ostlichen  Zentralalpen."     Neues  Jahrb.,  1895,  1,  221. 

Mineralogic  Composition. — All  igneous  rocks  are  silicate  rocks, 
and  their  essential  original  or  primary  constituents  are  either  silica 
or  silicates.  The  number  of  minerals  which  are  of  importance  in 
igneous  rocks  is  comparatively  small.  These  minerals  are  divided 
into  the  essential  or  c/we/.constituents,  which  are  necessary  to  that 
rock-type,  and  the  unessential  or  accessory  constituents,  whose 
presence  or  absence  does  not  alter  it.  Certain  of  the  latter  min- 
erals, such  as  apatite  and  zircon,  are  present  in  very  small  amounts 
in  almost  all  rocks,  while  others  rarely  occur.  In  some  rocks  an 
accessory  may  be  so  abundant  that  it  becomes  an  essential  and 
characteristic  constituent  for  that  variety,  for  example,  titanite  in 
syenite. 

In  many  rocks,  especially  in  those  of  the  nephelite-syenite 
group,  substitute  minerals  (Ger.  Stellvertreter)  occur  in  the  place  of 
some  normal,  essential  component,  and  thus  new  varieties  of  rocks 
are  formed.  Furthermore,  material  from  the  surrounding  rocks 


32  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

may  be  dissolved  in  the  melt  and  later  recrystallize  in  a  variety  of 
fortuitous  constituents. 

The  minerals  so  far  mentioned  are  primary  constituents  which 
crystallized  during  the  solidification  of  the  rock.  Besides  these, 
certain  secondary  constituents  were  formed  later,  some  of  them 
derived  entirely  from  outside  sources  through  gases  or  hot  solu- 
tions, others  produced  by  the  alteration  of  primary  minerals. 
These  secondary  minerals  may  be  subdivided  into  those  that  are 
crystalling  and  were  produced  by  post-volcanic  processes  of  replace- 
ment, and  those  that  are  predominantly  colloidal,  and  probably 
due  to  weathering. 

The  significance  of  a  mineral  may  be  quite  different  in  different 
rocks.  Thus  tourmaline,  which  occurs  instead  of  mica  in  many 
aplites,  is  undoubtedly  a  fumarolic  product  in  others.  Again, 
epidote  may  represent  a  constituent  taken  from  the  surrounding 
rocks,  or  it  may  be  a  secondary  product  derived  from  other 
minerals. 

The  constituents  of  igneous  rocks  may  be  classified  as  follows: 

I.  Primary  Minerals: 

(A)  Essential  constituents:     Quartz,  feldspars,  nephelite,  micas, 
pyriboles,1  and  olivine. 

(B)  Accessory  constituents:    Apatite,  xenotime,  titanite,  orthite, 
monazite,  chrysoberyl/  perofskite,  pyrope,  melanite,  magnetite, 
ilmenite,  chromite,  and  pyrrhotite. 

(C)  Substitute  constituents   (Ger.   Stellvertretende  Gemengteile) : 
Tourmaline,    eudialyte,    cancrinite,    catapleite,    leucite,    melilite, 
and  the  minerals  of  the  sodalite  group. 

(D)  Included  constituents    (Ger.   Aufgenommene  Gemengteile): 
Cordierite,  almandite,  the  epidotes,  staurolite,  corundum,  spinel, 
wollastonite,  and  some  pyriboles. 

II.  Secondary  Minerals: 

(A)  By  the  addition  of  material:     Tourmaline,  topaz,  scapolite, 
fluorite,  hematite,  and  pyrite.  4 

(B)  By    replacement:     Mica-like    minerals,    kaolin,    chlorite, 
serpentine,  prehnite,  garnets,   epidotes,  vesuvianite,   amphibole, 
titanite,  rutile,  anatase,  calcite,  quartz,  and  the  zeolites. 

(C)  By  weathering:     Primarily,  indeterminable  amorphous  sub- 
stances.    Also  calcite  and  quartz. 

1  A  general  term  covering  both  pyroxenes  and  amphiboles.     ALBERT  JOHANNSEN: 
"Petrographic  Terms  for  Field  Use."     Jour.  Geol,  XIX  (1911),  319. 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  33 

Certain  minerals  commonly  occur  with  certain  other  minerals,  or  in  certain  rocks. 
From  this  fact  a  number  of  laws  of  association  for  the  minerals  of  the  igneous  rocka 
have  been  formulated,  and  while  a  complete  review  of  these  cannot  be  given  at  the 
present  day,  the  following  appear  to  be  valid : 

1.  Quartz  never  occurs  as  a  true  constituent  in  silica-poor  rocks  that  are  rich 
in  the  alkalies.     It  is,  therefore,  not  present  with  nephelite  or  leucite.     Naturally  it 
is  more  common  in  silica-rich  rocks  than  in  those  that  are  basic.1 

2.  Potash-mica  does  not  occur  as  a  primary  constituent  in  association  with 
pyroxene  or  hornblende. 

3.  Olivine  is  rarely  associated  with  hornblende,  orthoclase,  or  primary  quartz. 

4.  Hornblende  is  usually  accompanied  by  titanite,  olivine  by  picotite,  and  melilite 
by  perofskite. 

5.  Biotite  is  not  abundant  in  soda-rich  rocks,  its  place  being  taken  by  a  soda- 
pyribole. 

""  6.  In  the  plutonic  series  from  granite  to  gabbro,  biotite  is  most  abundant  in 
rocks  that  are  richest  in  silica  and  the  alkalies.  When  these  are  less  in  amount, 
hornblende  takes  the  place  of  biotite,  and  this,  in  turn,  gives  place  to  augite  in  the 
subsilicic  rocks.  Granites,  therefore,  are  chiefly  mica-granites;  syenites  and  diorites, 
hornblende-bearing  rocks;  while  gabbros  usually  contain  augite. 

7.  In  soda-rich  rocks,  where  hornblende  and  pyroxene  are  zonally  intergrown, 
the  latter  usually  forms  the  outer  zone;  in  calcium-rich  rocks  the  hornblende  takes 
this  position. 

8.  In  persilicic  plutonic  rocks,  hornblende  is  green  in  more  cases  than  brown; 
in  the  subsilicic  rocks,  when  it  is  a  primary  mineral,  it  is  usually  greenish  brown  to 
brown. 

9.  In  the  extrusive  rocks,  hornblende  and  mica  appear  primarily  as  intratelluric 
minerals.     In  many  cases,  therefore,  they  show  magmatic  resorption,  and  the  more 
crystalline  the  rock,  the  farther  has  this  proceeded.     In  the  place  of  these  minerals, 
pyroxene  crystallizes  during  the  period  of  extrusion. 

10.  Soda-rich  pyriboles  appear  only  in  rocks  that  contain  more  soda  than  is 
necessary  for  the  formation  of  the  alkali-aluminium  silicates. 

11.  Melanite  and  most  of  the  titanium  and  zirconium  silicates  (except  zircon  itself) 
likewise  occur  only  in  soda-rich  rocks. 

12.  Nephelite,  leucite,  melilite,  and  the  minerals  of  the  sodalite  group  with  the 
exception  of  lazulite,  are  found  only  in  igneous  rocks. 

Distribution  of  the  Elements. — The  percentages  of  the  elements 
in  the  accessible  portion  of  the  earth's  crust  may  be  computed 
from  the  chemical  composition  of  the  various  rocks,  assuming  that 
the  quantitative  distribution  of  each  rock  type  is  known.  In 
such  a  computation  the  sedimentary  rocks  may  be  neglected,  since 
they  were  derived  from  the  destruction  of  igneous  rocks,  and  only 
the  latter  need  be  considered. 

The  percentages  of  the  elements  cannot  be  computed  from  a  random  collection 
of  rock  analyses,  no  matter  how  complete.  Such  collections  always  contain  many 
analyses  which  are  unimportant,  so  far  as  determining  the  average  composition  of 

1  CLARKE  (Bvll.  330,  U.S.G.S.,  p.  357,  and  Bull  616,  p.  425)  suggested  the  sub- 
stitution of  persilicic  and  subsilicic  for  acid  and  basic  when  applied  to  rocks,  the  latter 
terms  being  objectionable  on  account  of  their  definite  meaning  in  chemistry.  J. 


34  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

the  earth's  crust  is  concerned,  while  the  number  of  analyses  of  important  rocks  is  too 
small  to  be  representative  of  their  distribution  upon  the  surface.  As  was  shown 
above,  we  know  nothing  in  regard  to  the  primeval  crust  of  the  earth.  It  is  probable 
that  this  possessed  the  composition  of  granite,  since  granite  is  by  far  the  most  common 
intrusive  rock  and  occurs  in  the  largest  masses.  It  so  far  predominates  that  the 
addition  of  the  proper  proportions  of  other  plutonic  rocks  would  produce  but  a 
slight  change  in  the  chemical  composition  of  the  whole.  The  extrusive  rocks,  like- 
wise, are  of  comparatively  little  importance  when  compared  with  the  great  masses  of 
granite,  and  besides,  the  most  extensive  flows  are  of  quartz-porphyry,  consequently 
also  of  granitic  composition.  The  average  composition  of  our  earth's  crust,  therefore, 
is  unquestionably  much  nearer  the  composition  of  a  normal  granite  than  appears  to 
be  the  case  from  any  computation  yet  made,  for  each  of  these  is  based  directly  upon 
some  collection  of  analyses.1  The  agreement  in  the  results  of  the  various  computa- 
tions is  owing  to  the  fact  that  the  source  of  error  is  the  same  in  all. 

An  example  taken  from  an  extensive  granitic  area  will  serve  as  an  illustration. 
Normally  the  granite  is  so  abundant  that  its  composition  will  practically  correspond 
to  the  mean  composition  of  the  whole  region,  and  all  of  the  other  rocks  may  be  neg- 
lected. If,  however,  the  analyses  which  have  been  made  are  examined,  it  will  be  found 
that  the  very  uniform  granite  is  represented  by  only  one,  or  at  most  a  few,  analyses, 
the  majority  having  been  made  of  very  subordinate  dikes,  among  which  the  lampro- 
phyres  are  especially  abundant.  These,  however,  are  poorer  in  silica,  alumina,  and 
potash  than  the  granite,  and  therefore  contain  much  more  of  the  bivalent  metals. 
All  computations  based  upon  collections  of  analyses  show  the  same  error. 

.The  predominant  rocks  of  the  earth's  crust  are  orthoclase-bearing ;  those  carrying 
plagioclase  being,  by  comparison,  very  subordinate.  Potash,  therefore,  is  present  in 
greater  amount  than  soda.  This  preponderance  likewise  appears  in  the  composition 
of  the  sedimentary  rocks,  the  great  majority  of  argillites,  for  example,  carrying  two  or 
three  times  as  much  of  the  former  as  of  the  latter. 

The  composition  of  the  inner  core  of  the  earth  undoubtedly  differs  greatly  from  the 
mean  composition  of  the  crust.  This  is  evident  from  the  density  of  the  earth  itself  as 
compared  with  that  of  its  outer  shell.  The  former  being  about  twice  that  of  the 
latter,  forces^us  to  assume  the  presence  of  a  metallic  core. 

Chemical  Composition  of  Igneous  Rocks. — Chemical  analyses 
show  the  percentages  of  the  elements  present  in  rocks,  but  since 
most  rocks  are  aggregates  of  very  different  minerals,  usually  of 
complex  composition,  it  is  seldom  possible  to  compute  the  minerals 
present  in  a  rock  from  its  analysis.  The  computation  becomes 
still  more  difficult  if  percentages  of  the  different  minerals  are  to  be 
determined  quantitatively  from  the  analysis  without  a  previous 
microscopical  quantitative  determination  of  the  minerals  present. 
The  old  method  of  separately  analyzing  the  portions  soluble  and 
insoluble  in  hydrochloric  acid  gives  results  of  no  practical  value; 
nearly  all  silicates  are  attacked  by  concentrated  HC1,  the  amount 
dissolved  varying  greatly  according  to  the  length  of  action,  the  tem- 
perature, and  the  concentration  of  the  acid. 

While  valuable  and  interesting  data  may  be  obtained  from  a 

1  See  a  forthcoming  computation  by  A.  KNOPF:  Jour.  Geol.,  (1916).     J. 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  35 

study  of  the  chemical  analyses  of  igneous  rocks,  caution  is  neces- 
sary not  to  over-estimate  the  importance  of  a  single  analysis. 
There  is  always  danger  that  one  may  regard  as  most  reliable  those 
analyses  which  best  satisfy  preconceived  ideas. 

It  is  very  difficult  to  obtain  entirely  unobjectionable  material  for.  an  analysis 
which  is  to  represent  the  mean  composition  of  a  rock.  The  small  amount  of  material 
used  makes  a  total  analysis  of  a  coarse-grained  or  coarsely  porphyritic  rock  too 
much  like  that  of  some  specially  prominent  constituent.  Even  the  analysis  of  a  com- 
pact rock  is  not  necessarily  representative  of  the  entire  mass,  for  rocks  are  not  stoi- 
chiometrically  uniform  bodies,  as  are  crystals,  but  usually  differ  greatly  in  composi- 
tion in  different  places.  Thus  certain  basalts,  which  appear  perfectly  homogeneous  in 
fresh  condition,  under  the  influence  of  the  atmosphere  rapidly  develop  light-colored 
patches,  due  to  local  aggregations  of  nephelite.  Furthermore,  the  study  of  numerous 
thin  sections  from  a  single  igneous  mass  shows  the  irregular  distribution  of  the  individ- 
ual constituents.  A  rock-type,  therefore,  is  not  to  be  regarded  as  being  sharply 
defined  chemically,  but,  much  more,  as  varying  between  boundaries  not  too  narrow. 
The  strictly  chemical  tendency  of  modern  petrography  has  led  primarily  to  a  narrow 
view,  for  many  new  rock-types,  based  on  such  studies,  are  only  local  modifications 
not  entitled  to  an  independent  place  in  a  classification. 

The  chief  constituents  of  igneous  rocks  are  silica,  alumina, 
ferrous  and  ferric  oxides,  magnesia,  lime,  potash,  and  soda.  Be- 
sides these,  phosphoric  acid,  titanic  acid,  zirconium  oxide,  the  rare 
earths,  barium,  strontium,  and  water  are  usually  present  in  sub- 
ordinate amounts.  Furthermore,  in  certain  rocks,  manganese, 
chromium,  nickel,  cobalt,  tin,  uranium,  lithium,  caesium,  rubidium, 
sulphur,  arsenic,  antimony,  chlorine,  fluorine,  boron,  etc.,  may 
occur.  For  the  computation  of  chemical  rock-types,  however,  it  is 
sufficient  to  consider  the  first  eight  only. 

Since  all  igneous  rocks  are  silicate  rocks,  they  may  first  be  sub- 
divided according  to  their  silica  content.  This,  with  very  few 
subordinate  exceptions,  varies  between  78  per  cent,  and  40  per  cent. 
If  SiO2  is  more  than  65  per  cent,  the  rocks  are  called  acid,  if  from 
65  to  52  per  cent.,  intermediate  or  neutral,  and  if  less  than  52  per 
cent.,  basic.  It  must  be  borne  in  mind  that  these  designations  do 
not  correspond  to  the  chemical  distinctions  between  acid,  neutral, 
and  basic  salts.1  In  a  few  rare  cases,  such  as  in  ores  differentiated 
from  basic  igneous  rocks,  non-silicate  constituents  are  locally  so 
concentrated  that  they  become  essential  constituents.  These  ores 
are  practically  silicate-free,  but  hardly  fall  under  the  definition  of 
rocks. 

I  Cf.  footnote  page  33,  supra.  For  this  reason  the  terms  persilicic,  mediosilicic, 
and  subsilicic  are  to  be  preferred.  /. 


36  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

Natural  melts  are  not  made  up  of  any  random  combination  of 
elements.  There  are,  for  example,  silica-rich  rocks  high  in  alkali, 
but  no  equivalent  with  high  lime  and  (or)  magnesia  in  its  place. 
Likewise,  igneous  rocks  occur  which  consist  wholly  of  magnesium 
silicates  with  some  iron,  but  corresponding  rocks  rich  in  lime  and 
the  alkalies  are  unknown.  The  proportions  of  alumina  to  silica, 
alkalies,  or  lime,  rarely  exceed  that  necessary  to  form  feldspars. 
In  the  few  rare  cases  where  such  an  excess  occurs,  it  probably 
originated  by  the  assimilation  of  alumina-rich  sedimentary  rocks 
by  a  normal  melt,  and  does  not  represent  an  independent  rock-type. 

Rocks  whose  lime  content  is  less  than  the  sum  of  the  alkalies,  contain  between 
78  and  42  per  cent,  of  silica.  In  lime-rich  rocks  the  silica  does  not  exceed  66  per 
cent.,  and  in  rocks  whose  magnesia  is  in  excess  of  lime  it  is  usually  below  50  per  cent. 
In  sodic  rocks  the  silica  decreases  with  increasing  soda,  and  the  soda-orthoclase  rocks 
pass  over  into  nephelite  rocks;  a  high  lime  content  is  found  only  when  the  rocks  are 
rather  basic.  On  the  other  hand,  in  the-  usual  alkali-  and  silica-rich  melts  in  which 
potash  generally  predominates  over  soda,  a  decrease  in  the  amount  of  silica  is  ac- 
companied by  an  increase  in  the  lime  and  at  first  also  of  soda,  and  a  decrease  in  the 
potash.  This  brings  about  a  substitution  of  soda-lime-feldspar  for  orthoclase,  and 
with  a  still  further  decrease  in  silica,  a  preponderance  of  calcic  feldspar.  With  a 
decrease  in  silica,  therefore,  there  is  an  increase  in  the  lime,  magnesia,  alumina,  and 
iron  oxide  content,  and  a  decrease  in  the  sum  of  the  alkalies.  Lime  and  the  alkalies 
increase,  and  alumina  and  silica  decrease  in  rocks  in  which  lime  predominates  over  the 
alkalies,  that  is,  the  basic  minerals  which  are  poor  in,  or  free  from,  alkalies  and 
alumina  become  more  and  more  prominent.  Alkalies  and  alumina  decrease  along 
with  the  silica  in  rocks  with  more  magnesia  than  lime,  and  the  rocks  become  feld- 
spar-free. The  next  step  is  the  disappearance,  also,  of  the  lime,  and  the  extreme  end- 
member,  the  pure  orthosilicate  rock  olivinefels,  contains  only  the  silicate  of  magnesia 
and  iron. 

Physico-chemical  Laws  of  the  Magma. — The  facts  just  stated 
make  justifiable  the  conclusion  that  igneous  magmas  do  not  con- 
sist of  any  random  combination  of  substances,  but  that  the  differ- 
ent components  are  present  in  definite  proportions,  after  the 
manner  of  salts  in  a  solution.  Like  salts  in  solution,  also,  they 
are  probably  more  or  less  dissociated,  but  a  definite  balance, 
depending  upon  exterior  physical  conditions,  governs  their  mutual 
relationships. 

From  a  physico-chemical  standpoint,  magmas  are  natural, 
mixed  solutions  of  rather  complex  character.  In  most  cases  there 
were  present  in  the  original  melt,  besides  the  constituents  which 
can  be  recognized  in  the  solidified  rocks,  variable  amounts  of  the 
so-called  mineralizers — gases  which  acted  as  solvents  and  per- 
mitted the  rock  to  crystallize  more  readily.  Corresponding  to  the 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  37 

relationships  in  a  solution,  the  sequence  of  crystallization  of  the  indi- 
vidual minerals  was  determined  by  the  degree  of  their  solubility  in 
the  remainder  of  the  magma,  and  therefore  they  did  not  crystallize 
out  in  the  order  of  increasing  fusibility. 

The  textures  of  plutonic  rocks  show  that  the  magmas  were  very  fluid  during 
their  periods  of  crystallization.  In  contrast  with  contact-rocks,  which  are  to  be 
considered  later,  the  individual  constituents  of  the  plutonic  rocks  are  generally 
rather  poor  in  inclusions,  and  many  of  them  show  rather  perfect  crystallographic 
outlines,  characteristics  which  would  probably  not  have  appeared  had  the  magmas 
been  very  viscous  during  crystallization. 

It  is  of  interest  to  know  that  the  Neptunists  did  not  believe  it  possible  for  a  melt 
to  crystallize,  but  thought  it  solidified  in  an  amorphous  form.  The  solidified  masses 
were  supposed  to  undergo  a  later  crystalline  transformation  (hysterocrystallization; 
Gr.  Jtorepos,  later),  either  through  the  agency  of  circulating  water  or  by  a  gradual  inner 
molecular  change  in  the  solid  state,  somewhat  analogous  to  a  process  which  takes 
place  in  certain  metals.  The  latter  process  is  regarded,  at  the  present  time,  as  the 
cause  of  some  crystallo-genesis,  but  in  the  formation  of  rocks  it  is  probably  of  rather 
subordinate  importance. 

In  considering  the  processes  of  crystallization  according  to  the 
laws  of  solutions,  it  must  be  kept  in  mind  that  crystals  cannot 
separate  from  a  melt  so  long  as  its  temperature  is  higher  than  the 
crystallization  temperature  of  the  substance  at  a  given  pressure. 
Below  this  point,  the  crystallization  of  the  mineral  may  begin 
whenever  the  point  of  saturation  of  the  solution,  under  the  given 
physical  conditions,  is  exceeded.  A  very  considerable  lowering  of 
the  melting  point  is  often  noticed  in  complicated  mixtures.  Even 
in  the  simpler  silicate  melts,  this  reduction  maybe  as  great  at  400°. 

Most  rock-forming  minerals  occupy  less  volume  in  the  crystal- 
lized condition  than  in  the  fluid,  and  their  fusibilities  and  solu- 
bilities are  less  at  high  than  at  low  pressures.  Relaxing  the  pres- 
sure, therefore,  in  many  cases  has  the  same  effect  upon  crystals 
already  separated  as  increasing  the  temperature,  and  a  partial 
or  complete  re-solution  corrodes  or  resorbs  the  first  products  of 
crystallization,  as  is  characteristically  shown  by  the  quartz  or 
olivine  phenocrysts  of  certain  ,extrusive  rocks.  The  amount  by 
which  the  melting  point  is  increased  by  pressure  is  not  very  great, 
and  increase  in  pressure  does  not  raise  it  indefinitely,  for  there  is 
a  critical  point  above  which  the  substance  remains  fluid  under 
any  amount  of  pressure. 

The  phenomenon  of  resorption,  seen  in  the  corrosion  of  crystals 
of  quartz  or  olivine,  and  the  resorption  and  molecular  rearrange- 
ment of  hornblende  or  biotite,  occurs  chiefly  in  the  extrusive  rocks. 


38  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

It  is  the  more  complete  the  more  crystalline  the  rock,  and  the 
greater  the  loss  of  mineralizers  during  the  volcanic  eruption. 

The  first  minerals  to  separate  from  a  granite  magma  are,  in  general,  the  accessory 
constituents,  such  as  zircon  and  apatite,  which  are  least  fusible.  The  facts  that  they 
separate  early  and  are  present  in  minute  quantities,  "indicate  their  slight  solubility 
in  the  remaining  magma.  In  a  later  stage  of  the  solidification  of  the  rock  there  ap- 
pears, with  the  gradual  cooling,  a  saturation  for  the  ferromagnesian  silicates.  Thus, 
when  the  magma  has  sufficiently  cooled,  biotite  may  crystallize  and  in  its  solidification 
enclose  great  numbers  of  previously  formed  apatites  and  zircons.  The  separation  of 
the  biotite  goes  on  slowly  and  uniformly  since  it  becomes  more  and  more  insoluble 
with  further  decrease  in  temperature.  Gradually,  also,  the  point  of  saturation  for 
lime-bearing  plagioclase  is  reached,  and  this  now  separates  with  the  biotite.  The 
orthoclase  follows,  the  ferromagnesian  salts  having  become  by  this  time  completely 
exhausted.  Quartz  remains  liquid  at  relatively  low  temperatures,  since  it  is  most 
easily  soluble  in  the  mineralizers,  and  therefore  becomes  more  and  more  concentrated 
in  them  until  it  crystallizes  as  the  last  mineral.  The  mother-liquor  of  the  granite, 
therefore,  in  every  stage  of  its  solidification,  is  richer  in  silica  than  the  solidified  rock. 

In  a  similar  rnanner  the  change  of  the  sequence  of  crystallization  in  basic  rocks, 
in  which  the  bivalent  metal-rich  bisilicates  are  concentrated  in  the  mother-liquor, 
may  be  explained. 

With  the  crystallization  of  some  of  the  material,  the  composition  of  the  remaining 
magma  alters;  thus  the  glassy  base  of  a  persilicic  or  intermediate  rock  is  richer  in  silica 
and  the  alkalies  than  the  average  of  the  whole  rock.  This  gradual  alteration  in  the 
chemical  composition  of  the  melt  in  many  cases  leads  to  the  development  of  zones  in 
the  growing  crystals.  For  example,  a  plagioclase  feldspar  may  be  rather  high  in 
lime  in  the  first  period  of  its  crystallization,  but,  as  the  proportion  of  lime  in  the  magma 
decreases,  layers  progressively  richer  in  soda  are  added.  The  sequence  generally  is 
as  here  given,  the  border  zones  being  richest  in  soda  and  silica.  Similar  differences 
may  be  noticed,  not  infrequently,  on  comparing  the  plagioclase  phenocrysts  of  a  rock 
with  the  same  mineral  in  the  groundmass.  The  latter  is  always  richer  in  soda. 

Numerous  valuable  data  are  obtained  by  applying  the  theory 
of  solutions  to  the  separation  of  minerals  from  a  magma.  The 
natural  magmas,  however,  are  so  complex  that  the  overlapping 
of  the  different  processes  somewhat  obscures  the  significance  of 
each. 

The  study  of  eutectic  (Gr.  eu,  good,  re/crco*/,  builder)  mixtures 
has  produced  valuable  results.  These  mixtures  consist  of  two  or 
more  substances  in  a  single  solution  from  which  the  dissolved 
bodies  crystallized  simultaneously  in  intimate  intergrowths. 
A  eutectic  mixture  of  two  or  more  given  substances  has  a  constant 
composition,  and  changes  proportions  but  slightly  with  a  change 
of  pressure.  If  one  of  the  substances  is  present  in  greater  amount 
than  required  by  the  eutectic  mixture,  it  crystallizes  first  and  con- 
tinues to  do  so  until  the  composition  of  the  eutectic  is  reached. 
For  example,  the  eutectic  mixture  of  quartz  and  feldspar,  which 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  39 

is  seen  in  graphic  granite  (Fig.  13)  and  in  the  micropegmatitic, 
spherulitic,  and  glassy  groundmasses  of  quartz-porphyries, 
consists  of  about  75  per  cent,  of  feldspar  and  25  per  cent,  of  quartz. 
From  quartz-rich  magmas,  containing  a  mixed  solution  of  the  two, 
the  quartz  will  crystallize  before  the  feldspar,  producing  the  so- 
called  granulitic  texture  (PL  II,  Fig.  2).  If  the  amount  of  feld- 
spar exceeds  the  eutectic  mixture,  feldspar  will  separate  first,  pro- 
ducing the  granitic  texture  (PL  II,  Fig.  1).  A  reversal  in  the 
sequence  of  crystallization  of  feldspar  and  the  dark  constituents 
is  produced  in  a  similar  manner. 

It  was  formerly  thought  tl^at  magmas  solidified  at  exceedingly 
high  temperatures,  but  modern  experiments  have  shown  that  these 


FIG.  13. — Graphite   granite  from   Jekaterinburg,    Urals. 

are  far  below  those  which  may  be  reached  in  the  electric  furnace.1 
The  solidification  temperatures  are  especially  low  in  the  soda-rich 
rocks,  such  as  those  in  which  aegirite  and  arfvedsonite  occur  as 
the  first  products  of  crystallization. 

If  the  temperature  at  which  a  magma  is  saturated  with  a  certain 
mineral  lies  above  the  melting  point  of  that  mineral,  there  will 
occur,  in  place  of  crystallization,  a  sort  of  liquation,  that  is,  there 
will  appear  two  melts,  insoluble  in  each  other.  The  partial 
magmas  will  solidify  later,  perhaps  simultaneously,  perhaps  one 
after  the  other,  and  the  rock  will  show  schlieren".  If  one  of  the 

1  Observations  made  by  DAY  and  SHEPHERD  ("  Water  and  Volcanic  Activity," 
Bull.  Geol  Soc.  Amer.,  XXIV  (1913),  601)  show  the  temperature  of  the  Hawaiian  lavas 
to  be  between  1,070°  and  1,185°C.  at  the  surface.  J. 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


partial  magmas  is  much  more  fluid  than  the  other,  it  may  solidify 
with  sharply  defined  borders.     In  such  a  case  the  inclusion  may 


Peridotite  Chromite-rich  Chromite 

peridotite 

FIG.  14. — Schlieren  of  chromite  in  peridotite,  showing  transition  zones. 

have  the  appearance  of  a  later  intrusion,  for  the  more  mobile 
partial  magma,  having  penetrated  fissures  and  contraction  cracks 


Gabbro 


Reaction-rims, 
chiefly  garnet 


Nickeliferous 
pyrrhotite 


FIG.  15. — Dike  of  nickeliferous  pyrrhotite  in  gabbro,  showing  reaction  rims. 

in  the  portion  first  solidified,  intersects  it  in  dike-like  forms  and 
produces  the  impression  of  an  independent  body. 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  41 

The  first  relationship  is  the  more  widespread.  Schlieren-granites,  banded  gabbros, 
and  chromic  iron  segregations  in  peridotites  are  examples.  It  may  be  observed, 
especially  microscopically,  that  in  this  case  the  two  parts  are  generally  united  by 
narrow  transition  zones,  the  separation  not  having  been  complete  in  the  fluid  state 
(Fig.  14).  In  the  case  of  nickeliferous  pyrrhotite  segregations  in  gabbro,  the  ore  may 
have  solidified  after  the  beginning  of  crystallization  in  the  main  rock.  It  usually 
occurs  in  sharply  defined,  dike-like  forms,  in  many  cases  broken  into  disconnected 
parts,  as  though  by  movement  of  the  gabbro  during  the  last  stages  of  its  solidification 
(Fig.  15).  Transition  zones  between  the  ore  and  the  gabbro  are  not  present,  but  in 
many  places  the  latter  was  metamorphosed  at  the  contact  and  shows  reaction  rims. 

Action  of  Mineralizers. — It  has  been  shown  synthetically 
that  the  crystallization  of  the  different  essential  constituents  of 
igneous  rocks  depends,  more  or  less,  upon  the  physical  conditions 
under  which  the  cooling  takes  place.  Attempts,  such  as  those  of 
Michel-Levy,  to  produce  igneous  rocks  artificially,  have  shown 
that  it  is  easy  to  crystallize  the  constituents  of  subsilicic  rocks, 
primarily  olivine,  augite,  basic  plagioclase,  nephelite  and  leucite, 
by  slow  cooling  from  a  simple  silicate  melt.  Under  similar  condi- 
tions, however,  orthoclase,  quartz,  biotite,  and  hornblende  cannot 
be  made  to  crystallize. 

Quartz  and  orthoclase,  the  chief  constituents  of  the  persilicic 
rocks,  do  not  crystallize  from  simple  melts  because  their  melting 
points  are  more  than  100°  above  their  crystallizing  points. 
However  much  the  cooling  be  retarded,  the  amorphous  form,  that  is 
glass,  is  produced.  If,  however,  the  melting  points  are  reduced 
by  proper  means  to  the  temperatures  of  crystallization,  these 
minerals  also  pass  directly  from  the  melt  into  the  crystalline  condi- 
tion. In  nature  this  lowering  of  melting  points  is  produced 
primarily  by  the  gases  and  vapors,  especially  water  gas,  retained 
in  the  magma  by  high  pressure.  These  gases,  acting  as  solvents, 
keep  the  minerals  in  fluid  condition  until  the  temperature  is  far 
below  that  at  which  they  would  otherwise  solidify,  thereby 
making  possible  their  crystalline  development. 

These  miner alizers  are  of  importance  from  another  standpoint, 
for  hydroxyl-bearing  minerals,  such  as  mica  and  hornblende,  not 
uncommonly  occur  as  the  first  products  of  crystallization  from  the 
melt,  especially  in  the  silica-rich  rocks.  In  fact  the  micas  and 
amphiboles  cannot  crystallize  from  water-free  melts.  On  the 
other  hand,  the  most  essential  constituents  of  the  subsilicic  rocks, 
such  as  basic  plagioclase,  pyroxene,  and  olivine,  are  minerals 
whose  crystallization  temperatures  are  very  near  the  solidification 
point  of  the  melt,  consequently  rocks  which  contain  them  crys- 


42  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

tallize  readily  from  simple  melts,  and  even  though  the  mineralizers 
may  have  escaped  from  the  subsilicic  magma  during  its  extrusion, 
the  rocks  do  not  necessarily  develop  a  glassy  habit  but  may  even 
be  holocrystalline. 

A  comparison  of  plutonic  rocks  with  surface  flows  clearly  shows  the  significance 
of  the  mineralizers.  In  the  former,  they  remained  in  the  magma  on  account  of  the 
pressure  of  the  overlying  strata,  in  the  latter  they  escaped  as  clouds  of  vapor  when  the 
pressure  was  removed  from  the  melt.  The  change  in  the  physical  conditions  under 
which  the  crystallization  took  place  is  very  distinctly  shown  in  the  appearance  of 
the  rocks,  and,  as  might  be  expected  from  what  has  been  said  above,  this  is  most 
marked  in  the  silicic  rocks. 

Granitic  rocks  must  have  solidified  at  a  considerable  distance  beneath  the  surface 
of  the  earth,  and  the  crystalline  development  of  the  biotite,  orthoclase,  and  quartz, 
indicates  that  mineralizers  were  present,  while  the  uniformity  of  the  texture  shows 
that  the  solidification  took  place  under  conditions  of  uniform  cooling. 

If  an  upward  movement  of  the  magma  relieved  the  stress  and  permitted  partial 
escape  of  the  enclosed  gases  and  vapors,  this  fact  is  expressed  in  the  texture  of  the 
rock,  which  then  appears  porphyritic,  and  the  larger  intratelluric  (Lat.  intra,  within, 
tellus,  earth)  phenocrysts  of  feldspar  and  quartz,  which  were  formed  under  the  uniform 
conditions  of  the  interior  of  the  earth,  stand  out  from  the  more  or  less  dense  ground- 
mass,  whose  texture  expresses  the  later  conditions  of  formation.  Finally,  if  the  melt 
reached  the  surface  as  a  lava-stream,  all  of  the  mineralizers  were  completely  lost. 
If  the  magma  was  that  of  a  normal  granite,  the  constituents  could  not  pass  from  the 
fluid  phase  into  the  crystalline,  but  that  portion  which  did  not  crystallize  in  the  deeper 
zone,  solidified  as  glass.  In  many  cases  the  change  in  the  condition  of  the  melt  caused 
it  to  react  upon  the  previously  crystallized  minerals,  which,  though  stable  under  the 
conditions  of  formation,  now  became  unstable,  and  were  either  entirely  dissolved,  or 
corroded  and  embayed.  Thus  the  hydroxyl-bearing  micas  and  hornblendes  may  have 
been  entirely  destroyed  or  magmatically  resorbed,  and  augite  may  have  taken  their 
place.  As  a  result,  the  deep-seated  granite  differs  essentially  in  texture  and  mineral 
composition  from  the  chemically  equivalent,  extrusive  rhyolite  (liparite). 

On  the  other  hand,  there  is  no  such  marked  difference  between  gabbro  and  basalt, 
which  are  silica-poor,  and  normally  consist  of  labradorite,  augite,  and  some  olivine. 
Even  the  extrusive  members  of  this  group  show  a  pure  crystalline-granular  develop- 
ment without  an  indication  of  porphyritic  texture,  and  under  certain  conditions 
may  be  quite  coarsely  crystalline,  for  example,  in  massive  flows  which  permit  slow 
cooling. 

But  even  the  less  massive  sheets  of  these  subsilicic  rocks  appear  holocrystalline 
under  the  microscope,  glassy  rocks  being  practically  wanting.  This  holocrystalline 
development  of  relatively  thin  sheets  of  trap,  seen  at  the  Wener  Sea  in  Sweden,  in  the 
Hebrides,  and  in  Iceland,  stands  in  marked  contrast  with  the  common  occurrence  of 
holohyaline  rhyolite,  such  as  that  in  the  Glashiittental  near  Schemnitz  in  Hungary, 
or  in  the  Yellowstone  Park. 

The  mineralizing  agents  ordinarily  do  not  enter  into  the  com- 
position of  the  solidified  igneous  rocks,  but  pass  off,  either  quickly 
in  the  form  of  great  clouds  of  vapor  during  a  volcanic  eruption,  or 
slowly  and  uniformly  deep  within  the  earth,  as  they  are  excluded 
by  the  crystallizing  minerals,  to  diffuse  through  the  highly  heated 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  43 

country-rock  and  renew  their  mineralizing  activity.  By  this 
means  they  become  of  great  importance  in  the  alteration  of  the 
country-rock,  and  are  therefore  classed  as  agents  of  contact- 
metamorphism. 

Finally,  mineralizers  play  an  important  part  in  the  formation 
of  the  so-called  complementary  dikes — subsequent  extrusions  of 
differentiated  rocks — especially  when  these  are  rich  in  silica  and  the 
alkalies.  The  differentiation  rocks,  therefore,  may  be  very  coarse- 
grained like  the  pegmatites,  or  the  texture  may  be  more  or  less 
porous  with  cavities  filled  with  well-developed  crystals,  indicative 
of  their  growth  in  the  presence  of  concentrated  mineralizers. 

It  is  noteworthy  that  different  natural  magmas  contain  very  different  amounts  of 
mineralizers.  As  just  mentioned,  those  that  are  rich  in  silica  and  the  alkalies  contain 
great  amounts,  while  subsilicic  rocks,  rich  in  the  alkaline  earths,  contain  but  little. 
This  is  shown  by  the  intensity  of  the  contaet-metamorphism,  by  the  development  of 
pegmatite,  and  by  other  evidence.  The  country-rocks  of  granites,  and  more  especially 
of  the  alkali-rich  nephelite-syenites,  may  be  altered  to  a  distance  of  many  kilometers, 
and  they  are  usually  cut  by  numerous  pegmatites  filled  with  interesting  minerals. 
On  the  other  hand,  in  the  much  rarer  intrusive  rocks  which  are  poor  in  silica  and  rich 
in  the  bivalent  elements,  both  contact-metamorphism  and  pegmatites  are  much  less 
common. 

The  relation  between  mineralizers  and  the  character  of  the  rocks  is  further  shown 
by  the  fact  that  subsilicic  igneous  rocks,  recognizable  by  their  mineral  composition 
and  preserved  remnants  of  textures  as  having  been  originally  gabbros,  traps,  or  even 
labradorite-porphyrites,  are  completely  recrystallized  at  the  contact  with  granites  or 
nephelite-syenites.  Without  having  undergone  any  essential  change  in  their  chemical 
character,  they  are  altered  to  rocks  composed  of  entirely  different  minerals.  Thus 
gabbros  or  porphyrites,  which  originally  crystallized  from  magmas  poor  in  mineral- 
izers, were  altered  to  amphibolites  and  eclogites  by  the  action  of  mineralizers  emanat- 
ing from  alkalic  rocks.  If  the  original  magma  had  been  rich  in  mineralizers,  rocks 
similar  in  mineral  composition  to  eclogites  and  amphibolites  undoubtedly  would 
have  resulted  directly  from  the  first  consolidation. 

The  effect  of  mineralizers  on  the  composition  of  a  rock  is  undoubtedly  seen  in 
these  contact-rocks.  In  some  cases,  however,  the  mineralizers  did  not  act  alone  but 
were  aided  to  some  extent  by  the  great  pressures  which  acted  upon  the  magma  during 
the  orogenic  movements,  as  is  well  shown  by  the  phenomenon  of  piezocrystallization 
which  is  described  in  a  later  chapter.  Of  especial  interest,  in  this  connection,  are 
certain  peculiar  jadeites  which  occur  in  serpentine.  Chemically  they  consist  of  one 
part  nephelite  and  one  part  albite,  but  their  specific  gravity  is  much  higher  than  an 
aggregate  of  these  two  minerals  [Jadeite  (sp.  gr.  3.3)  2NaAlSi2O6  =  nephelite 
(sp.  gr.  2.6)  NaAlSiO4  +  albite  (sp.  gr.  2.6)  NaAlSiaOg]. 

The  amount  of  tuff  occurring  in  association  with  extrusive 
rocks  depends  to  a  certain  extent  upon  the  amount  of  mineralizers 
present  in  the  magma;  the  more  gas,  the  more  dust-like  material 
hurled  forth  by  its  escape.  In  fact,  an  excessive  amount  of  gas 
may  lead  to  a  great  explosion,  and  cause  the  shearing  of  a  smooth 


44  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

passage  through  the  earth's  crust  and  the  ejection  of  much  finely 
divided  material,  which  falls  back  and  is  deposited  in  and  around 
the  mouth  of  the  crater.  The  tuff  produced  by  such  explosive 
eruptions  is  either  of  the  character  of  the  most  silicic  of  the  igneous 
series  or,  strangely  enough,  of  the  most  basic  of  the  differentiation 
rocks.  The  rhyolite-tuff  at  Ries  near  Nordlingen,  Bavaria,  the 
enormous  masses  of  quartz-porphyry-tuff  at  Bozen  in  South  Tyrol, 
and  the  pumiceous  tuff  with  the  composition  of  a  soda-rich 
trachyte — the  so-called  trass — in  the  Brohl  valley  near  Andernach, 
are  examples  of  the  first  class,  while  the  melilite-basalt-tuffs  of 
the  Swabian  Alb  and  the  diamondiferous  kimberlite  of  South 
Africa  are  examples  of  the  second. 

The  subsilicic  igneous  rocks,  with  the  exception  of  these  basic 
differentiation  rocks,  were  very  poor  in  mineralizers.  This  is  shown, 
for  example,  by  the  lava  at  Kilauea,  which  gives  off  no  vapors,1 
and  by  numerous  basaltic  "  Quellkuppen "  (Fig.  11)  which  are 
practically  unaccompanied  by  tuff. 

Magmatic  Differentiation. — In  many  cases  the  texture  and 
composition  of  an  igneous  body  is  not  uniform  throughout,  but 
within  it  there  are  parts  which  differ  decidedly  in  composition  from 
the  main  mass,  yet  are  united  to  it  by  all  possible  transitions. 
Such  irregular  masses  are  spoken  of  as  schlieren.  They  may  be 
subdivided  into  (a)  Constilution-schlieren,  originating  in  the  com- 
plete splitting  up  of  a  previously  uniform  magma;  (b)  Differ  entia- 
tion-schlieren,  produced  by  a  gathering  together  of  the  material 
first  crystallized;  (c)  Resorption-schlieren,  originating  in  the  partial 
assimilation  of  included  fragments  of  the  country-rock  (the 
dark  basic  patches  (Putzeri)  occurring  in  many  granites  probably 
belong  here) ;  (d)  Injection-schlieren  or  schlieren-dikes,  which  were 
formed  during  the  last  stages  of  the  solidification  of  the  rock  by  the 
injection  of  molten  material  into  shrinkage  fissures.  Finally  (e) 
the  last  remnant  of  the  melt  itself,  which  in  general  is  characterized 
by  an  enrichment  in  mineralizers,  may  separate  into  schlieren-like 
masses  of  pegmatite.  These  are  called  hysterogenic  schlieren 
(Gr.  iforepos,  later,  yiyvo^ai,  to  be  born). 

Magmatic  differentiation  in  igneous  rocks  may  be  on  a  large 
or  on  a  small  scale.  Numerous  great  granitic  masses  consist  of 
unlike  rocks,  and  in  many  cases,  especially  in  stocks,  the  silica  and 

1  For  the  contrary  see  DAY  and  SHEPHERD,  Op.  cit.     J. 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  45 

alkali  content  gradually  decrease  from  the  center  to  the  border, 
so  that  there  are  all  transitions  from  a  silicic,  orthoclase-quartz 
rock  at  the  center  to  a  subsilicic  aggregate  of  plagioclase  and 
pyroxene  at  the  border  (Fig.  16).  In  spite  of  the  great  chemical 
and  petrographical  differences  between  the  gabbro-like  borders 
and  the  granite  interior,  all  the  transitions  are  considered  fades 
of  the  granite,  being  parts  of  the  same  geologic  unit. 

Furthermore,  the  various  lava  flows  of  a  petrographic  province 
show  chemical  and  petrographical  differences,  indicating  a  very 
gradual  change  of  the  magma  from  subsilicic  to  silicic.  This  is 
one  of  the  chief  arguments  in  favor  of  StubeFs  theory,  suggesting 


Two-mica-        Biotite-    Amphibole-    Diorite    Gabbro 
granite  granite          granite 

FIG.  16. — Magmatic  differentiation  in  a  granite  stock. 

that  magmatic  differentiation  took  place  in  one  of  the  hypothetical 
peripheral  magma  basins  in  a  manner  analogous  to  that  in  the 
granite  stock,  most  of  the  insoluble  constituents,  in  this  case  the 
lime-iron  silicates,  being  concentrated  against  the  cooler  roof, 
while  silica  and  the  alkalies  increased  with  depth.  The  first  lava 
extruded  having  come  from  the  upper  portion  of  the  basin  and 
succeeding  flows  from  deeper  and  deeper  levels,  the  composition 
of  the  erupted  rocks  becomes  progressively  more  silicic. 

The  silica-poor  melt,  which  segregated  in  the  higher  levels,  was  undoubtedly 
specifically  heavier  than  the  silica-rich  magma  of  the  deeper  zones,  yet  a  study  of  the 
igneous  rocks  shows  that  specific  gravity  plays  no  important  role  in  the  arrangement 
of  the  constituents  even  in  the  case  of  the  heavier  ores,  great  masses  of  ilmenite,  mag- 
netite, chromite,  and  presumably  also  of  platinum,  occurring  in  schlieren-like  segre- 
gations and  not  at  the  base  of  the  igneous  body  with  which  they  are  associated. 

In  many  cases  of  this  kind  the  nature  of  the  igneous  rocks,  as  well  as  of  their 
schlieren,  excludes  the  assumption  that  the  magma  was  so  viscous,  at  the  time  of  the 
formation  of  the  schlieren,  that  separation  could  not  take  place  by  specific  gravity. 


46  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

Other  physical  influences,  therefore,  perhaps  electric  or  magnetic,  must  have  counter- 
acted the  effects1  of  specific  gravity. 

This  phenomenon  deserves  especial  attention  because  differentiation  by  specific 
gravity  has  undoubtedly  taken  place  at  the  earth's  center,  the  inner  core  being  much 
denser  than  any  known  igneous  rock.  The  specific  gravity  of  the  whole  earth  (5.2) 
is  twice  that  of  the  known  crust;  its  inner  parts,  therefore,  must  contain  much  heavier 
substances  than  those  which  compose  its  shell.  It  was  formerly  supposed  that  the 
molecular  weight  of  the  individual  constituents  was  the  cause  for  this  difference. 
The  earth's  crust  was  thought  to  be  built  up  of  zones,  the  outer  one,  of  lithium- 
bearing  granite,  passing  into  rocks  containing  soda  and  potash.  Deeper  down,  strata 
of  basic  magmas  graded  into  pure  magnesia-iron  silicate  melts  and  finally  into  the 
supposed  metallic  core.  These  assumptions  had  no  field  observations  to  support 
them,  and  were  purely  theoretical. 

Differentiation  on  a  small  scale  is  seen  where  inclusions  occur 
in  silica-rich  igneous  rocks.  Through  the  partial  solution  of  the 
constituents  of  these  inclusions  the  equilibrium  of  the  magma  is 


FIG.  17. — Gabbro-diorite  border  facies  of  granite  with  schlieren  due  to   inclusions. 
(From  Webern,  Neunkirchner  Hohe,  Odenwald.) 

disturbed  and  differentiation  takes  place,  in  many  cases  causing 
distinct  banding  (Fig.  17).  The  same  phenomenon  occurs  on  a 
more  extensive  scale  where  the  inclusions  are  large.  In  such  cases, 
in  the  otherwise  haphazard  (richtungslos)  granular  granite,  these 
fragments  are  surrounded  by  zones  of  gneiss  showing  parallel 
structures  due  to  magmatic  differentiation. 

A  banded  development,  much  like  bedding,  is  found  in  many 
subsilicic  igneous  rocks,  especially  in  gabbros  (Fig.  18),  and  banded 
areas  also  occur  in  nephelite-syenites  and  peridotites. 

Many  intrusive  rocks  show  the  effects  of  magmatic  differentia- 
tion in  their  narrow  border  zones,  the  variation  being  especially 
great  where  silica-rich  rocks,  like  granites,  have  broken  through 


THE  COMPOSITION  OF  IGNEOUS  ROCKS 


47 


rocks  rich  in  lime.  In  such  cases  the  whole  series  of  plutonic  rocks 
may  appear  within  a  relatively  short  distance,  the  granite  of  the 
central  mass  grading  into  pyroxenite  and  peridotite  at  the  contact. 

Local  differentiation  processes  of  this  kind  had  a  limited  period  of  action,  for  they 
ceased  upon  the  crystallization  of  the  magma.  The  transition  diorites  or  gabbros, 
therefore,  were  usually  formed  under  abnormal  conditions,  and  as  a  consequence 
differ  from  normal  types  in  many  particulars.  The  magma  nearest  the  contact  dis- 
solved considerable  lime.  The  melt,  therefore,  was  lime-rich  and  silica-poor  and,  at 
the  same  time,  carried  a  little  magnesia  and  iron,  and  much  alkali.  Such  a  chemical 
combination  does  not  occur  in  igneous  rocks;  it  therefore  represents  an  unstable  con- 
dition of  the  magma.  Diffusion  must  immediately  have  set  in  and  have  produced  an 
increase  in  magnesia  and  iron  at  the  periphery  of  the  mass  and  a  decrease  in  the  alkalies, 


FIG.  18.— Banded  gabbro.     Isle  of  Skye.     (After  A.  Geikie.) 

the  latter  enriching  the  granite  itself.     Thus  the  tendency  of  the  magma  toward 
equilibrium,  represented  by  the  normal  igneous-rock  series,  again  appears. 

These  endogenic  contact  phenomena  are  generally  much  less  distinct  at  the  contact 
of  granitic  rocks  with  slates,  for  the  difference  between  the  composition  of  the  dissolv- 
ing and  the  dissolved  material  is  not  so  great.  Yet  even  here,  in  many  cases,  a  close 
examination  will  show  that  the  adjacent  rock  has  been  dissolved,  and  garnet,  cordi- 
erite,  and  aluminium  silicates  or  corundum  and  spinel  will  be  found  in  the  intrusive. 
The  assimilation  zone  at  the  contact  with  the  slate,  also,  is  much  less  silicic  than  the 
granite  itself,  that  is,  it  is  richer  in  mica  and  poorer  in  orthoclase,  the  latter  usually 
occurring  only  as  large,  sporadic  phenocrysts. 

Basic  border  zones  of  this  kind  are  common,  and  most  of  them 
are  undoubtedly  due  to  an  assimilation  of  the  adjacent  rock.  In 
some  places  they  are  very  poor  in  quartz  but  rather  rich  in 
orthoclase  (mica-syenite  in  part),  elsewhere  the  feldspar  is  almost 
wholly  plagioclase  (mica-diorite  in  part).  In  both  cases,  however, 


48 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


the  rocks  are  more  melanocratic  (Gr.  /zeXas,  black,  KpaTetv,  to 
dominate)  than  the  corresponding  normal  rock  and,  except  for 
their  granular  texture,  very  closely  resemble  lamprophyres. 

The  significance  of  border  zones  in  which  silica  and  the  alkali- 
rich  minerals  predominate  is  entirely  different.  Such  rocks  are 
light-colored  or  leucocratic  (Gr.  Xeu/c6s,  white)  and  correspond  in 
composition  to  the  aplites.  They  are  found,  for  example,  around 
the  granites  of  the  central  Alps,  and  are  especially  typical  of  the 
tin-granite  of  the  Erzgebirge,  where  they  usually  show  a  coarse- 
grained, pegmatite-like  development. 

The  leucocratic  border  rocks  show  that  the  mineralizers  were  exceptionally  con- 
centrated in  the  aplitic  magma,  making  it  extremely  mobile  and  causing  it  to  penetrate 

the  shattered  contact-rocks  with  great  ease, 
in  many  cases  saturating  them  completely. 
The  intruded  rocks  in  injection-schists1 
are  generally  aplite  or  pegmatite  veins. 

Differentiation  rocks  are  found  not 
only  around  large  masses  of  plutonic  rocks, 
but  also  very  commonly  as  selvedges  of 
relatively  subordinate  dikes.  The  lampro- 
phyric  and  aplitic  borders  of  the  dikes 
(Fig.  19)  so  commonly  intersecting  granite, 
however,  cannot  be  explained  on  the  basis 
of  assimilation  of  the  adjacent  rock.  Here 
the  magma  suffered  internal  differentiation, 
In  the  case  of  the  aplite,  the  borders  not 
rarely  are  more  completely  crystalline  than 
the  dike  itself,  for  example  in  the  quartz- 
porphyry  dikes  near  Regensburg  where  the 
selvedges  are  of  normal  granular  aplite. 
But  an  increase  in  the  crystallinity  of  the 
border  zones  is  found  only  where  dikes 
were  intruded  immediately  after  the  in- 
trusion of  the  main  mass  itself  and  while 
it  was  still  extremely  hot.  Later  dikes  and 

those  extending  farther  from  the  parent  magma  are  rich  in  glass  and  fine-grained  at 
the  borders. 

Complementary  Dikes. — The  differentiation  processes  so  far 
mentioned,  which  all  took  place  where  the  intruded  mass  itself 
solidified,  are  called  laccolithic  differentiations.  Contrasted  with 
these  are  the  abyssal  differentiation  processes  which  took  place 
deep  within  the  earth,  and  which  produced,  within  peripheral 
magma  basins,  all  normal  magmas  and  the  various  rocks  of  a  single 

1  The  term  injected-schist  is  frequently  used.     Injection-schist  is  better,  the  granite 
being  the  injected  rock.     J. 


Granite 


Granite- 
porphyry 


Syenite 
porphyry 


with  basic  selvedges 

FIG.  19. — Differentiated  dikes  in  the 
Trusetal,  Thuringen.     (After  H.  Biick~ 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  49 

petrographic  province.  Here  also  belong  the  differentiation  proc- 
esses which  produced  the  complementary  or  satellite  (Ger.  Gangge- 
folgschaft,  literally  dike-retinue)  rocks  which  in  so  many  cases  fol- 
lowed the  plutonic  rocks,  and  usually,  though  not  always,  cut 
them  in  the  form  of  dikes.  The  term  Ganggesteine  (dike  rocks), 
originally  used  by  Rosenbusch,  is  not  an  accurate  term  for  all 
occurrences,  and  it  is  perhaps  better  to  substitute  differentiation 
rocks,  since  they  always  show  certain  chemical  variations  from  the 
normal  magma. 

A  great  variety  of  differentiation  rocks  is  associated  with 
nephelite-syenite,  its  magma  being  especially  subject  to  splitting. 
Most  of  these  differentiation  products,  however,  properly  should 
not  be  called  rocks,  for  they  are  much  too  unimportant  and  only 
of  local  significance.  Naming  each  mineral  combination  produces 
great  confusion  in  the  nomenclature  and  causes  a  loss  of  per- 
spective by  magnifying  the  importance  of  these  isolated  occur- 
rences, making  the  relationships  appear  much  more  confusing  than 
they  actually  are.  The  relation  of  the  differentiation  products 
to  the  parent  rocks  is  much  more  easily  understood  in  the  granite, 
series  than  in  the  nephelite-syenite,  consequently  they  may  be, 
used  as  types  of  the  complementary  dikes. 

Rocks  of  the  composition  of  the  main  igneous  mass  are  rare  among  the  succeeding 
dike-intrusions.  From  their  undifferentiated  condition  they  are  called  aschistic 
(Gr.  a<7x«"-6s,  unsplit)  rocks.  The  dikes  cutting  the  parent  rock  are  usually  somewhat 
lighter  or  somewhat  darker  than  the  latter,  and  represent  limiting  cases  of  differen- 
tiation. Such  dikes  are  called  diaschistic  (Gr.  Stao-xiaros,  cleaved).  When  light  in 
color  they  belong  to  the  aplitic  differentiation  series,  when  dark  to  the  lamprophyric. 

The  intimate  relationship  between  the  differentiation  rocks  of  the  granite  family 
are  shown  in  Fig.  20.  Aplite  (Gr.  oTrXoos,  simple,  therefore  more  properly  called  haplite) 
occurs  when  there  is  an  increase  in  silica  and  the  alkalies,  and  a  decrease  or  complete 
lack  of  dark  constituents.  Lamprophyres  (Gr.  \anirp6s,  brilliant,  on  account  of  the 
appearance  of  the  mica-rich  varieties)  occur  when  an  increase  in  the  bivalent  bases 
produces  an  increase  in  the  amount  of  the  dark  constituents.  With  these  changes  the 
proportions  of  the  alkalies  are  altered,  orthoclase  rocks,  with  predominating  potash, 
resulting  on  the  one  hand,  and  potash-poor  plagioclase  rocks  on  the  other.  The  ortho- 
clase series  begins  with  ordinary,  fine  equigranular,  orthoclase-rich,  biotite-free  aplite, 
and  leads  through  the  transition  member  semi-aplite,  which  contains  a  little  biotite, 
to  granite-porphyry.  From  aplite  to  granite-porphyry  the  rocks  approach  nearer  and 
nearer  the  composition  of  normal  granite.  Under  certain  conditions  they  may 
develop  a  lamprophyric  facies  by  becoming  richer  in  the  ferromagnesian  constituents. 
In  this  case  they  become  darker,  and  from  brownish-black  minette  without  feldspar 
]phenocrysts,  there  is  a  rapid  transition  to  proterobase  with  practically  no  orthoclase, 
and  therefore  rather  representing  a  subsilicic,  plagioclase-rich,  granitic  lamprophyre. 

With  still  more  plagioclasje,  the  aplites  become  true  plagioclase  rocks;  thus  many 
megascopically  normal  aplites  are  rich  in  albite.  Further,  the  semi-aplitic  alsbachite 


50 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


contains  a  lime-soda-plagioclase  as  its  chief  feldspar,  and  the  lamprophyres  of  this 
series  are  generally  still  more  calcic.  On  the  other  hand,  proterobase,  which  normally 
carries  a  calcic  feldspar,  not  infrequently  contains  albite  as  the  only  plagioclase,  but 
this  is  usually  secondary,  being  derived  from  the  calcic  plagioclase  by  metamorphism. 
Finally,  both  end-members  of  these  differentiation  rocks  are  feldspar-free.  At  one 
extreme  the  aplites  become  aggregates  of  pure  quartz,  numerous  quartz  dikes  being 
true  differentiation  rocks,  while  at  the  other  the  proterobase  loses  its  feldspars,  and 
the  resulting  picrite-porphyrite,  which  consists  chiefly  of  olivine  and  pyroxene,  repre- 
sents the  most  basic  of  the  granitic  differentiation  rocks. 

Pikriteporphyrite 
proterobase 


Lamprophyre 


Increase  in 
CaO,MgO.FeO 


Increase  in 
Si02  and  the  Alkalies 


Aplite 
^Quartz 

Predominant 
Orthoclase  Plagioclase 

FIG.  20. — Diagram  showing  the  relationships  between  the  granitic  differentiation 
rocks.     (After  B.  Sandkiihler.) 

The  silicic  differentiation  rocks,  especially  aplite,  were  derived 
from  melts  rich  in  mineralizers,  and  in  many  cases  contain  much 
tourmaline;  normal  tourmaline-granite,  like  muscovite-granite, 
being  a  form  of  aplite.  All  mineralizer-rich  magmas  are  very  sensi- 
tive to  changes  in  physical  conditions  during  their  consolidation, 
consequently  when  an  aplitic  magma  was  extruded  at  some  distance 
from  the  volcanic  center,  or  when  its  period  of  intrusion  came  long 
after  that  of  the  main  mass,  the  rock  developed  into  aplite- 
porphyry,  resembling  quartz-porphyry,  or  pitchstone-porphyry 
rich  in  glass.  The  largest  known  occurrences  of  pitchstone  were 
derived  from  aplitic  magmas  which  solidified  as  glass  and  they 
carry  the  greater  part  of  the  mineralizers  in  solid  solution.  From 


THE  COMPOSITION  OF  IGNEOUS  ROCKS 


51 


chemical  analyses  of  such  natural  glasses  some  idea  of  the  enormous 
amount  of  gas  which  was  contained  in  the  magma  may  be  obtained. 
Thus  fresh  pitchstone  from  the  neighborhood,  of  Meissen  contains 
as  much  as  10  per  cent,  by  weight  of  water. 

The  behavior  of  the  lamprophyres  is  entirely  different.  Their 
magmas  originally  were  probably  less  rich  in  mineralizers,  conse- 
quently much  less  fluid,  than  the  aplitic  magma.  The  aplites, 
therefore,  in  many  cases  branch  into  fine  veinlets  (Fig.  21),  while 
lamprophyres  form  stock-like  masses  or  very  simple  dikes  with 
broad  and  blunt  ends  (Fig.  22).  Aplite  is  crystalline  only  within 


FIG.  21. — Aplite  dike  in  amphibolite.     Untersulzbachtal,  near  the  Grossvenediger. 

the  zone  of  contact-metamorphism ;  the  lamprophyres  in  many 
cases  show  no  alteration  in  their  character  even  when  rather  far 
distant  from  the  igneous  center,  or  when  within  entirely  unaltered 
sediments,  and  they  show  a  glassy  development  only  under 
exceptional  conditions. 

In  the  porphyritic  lamprophyres  the  only  true  phenocrysts  are 
mica,  hornblende,  and  (or)  pyroxene,  yet  here  and  there  excep- 
tionally large,  phenocryst-like  individuals  of  orthoclase  and  quartz 
are  found.  If  these  larger  crystals  are  examined  carefully,  it  will 
be  seen  that  they  differ  from  true  phenocrysts  in  being  more  or 
less  rounded,  as  though  fused  at  the  edges  (Fig.  23),  an  indication 


52 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


that  they  were  formed  deep  within  the  earth  in  the  granitic  magma 
itself,  and  were  removed  from  it  by  the  lamprophyres  during  their 
extrusion.  Such  primeval  inclusions  are  extremely  numerous  in 
some  lamprophyres  and  in  true  basalts,  olivine  inclusions  in 
particular  being  rarely  wanting  in  the  latter. 

Aplites  and  pegmatites  rarely  contain  inclusions,  but  they  are 
peculiar  in  other  ways.  Most  important  is  their  high  mineralizer 
content,  to  which  reference  has  already  been  made.  The  effect 
of  the  mineralizers  is  to  make  the  magma  a  strong  solvent  of  the 
adjacent  rocks,  so  that  the  constituents  of  the  latter  are  incorpo- 
rated within  it.  This  is  especially  marked  where  the  country- 


FIG.  22. — Lamprophyre  dike  in  granite.  FIG.  23. — L amprohyre  with 

Norway,  Maine.  rounded     orthoclase     phenocrysts. 

Gailbach,  near  Aschaffenburg. 

rocks  are  entirely  different  chemically.  It  may  be  seen  in  the 
staurolite-,  paragonite-,  and  disthene-paragonite-schists  from  the 
neighborhood  of  St.  Gotthard,  in  the  garnet-rich  aplites  from  many 
localities,  and  in  the  zoisite  rocks  from  Weissenstein  in  the 
Fichtelgebirge. 

Pegmatites  were  usually  the  first  of  the  complementary  dikes 
to  form,  being  forced  into  the  contraction  fissures  of  the  main  rock 
during  the  last  stages  of  its  crystallization.  The  minerals  of  the 
pegmatites,  not  rarely,  are ,  simply  continued  growths  of  the  con- 
stituents of  the  intrusive  rock  itself,  so  that  welded  dikes  are  formed 
when  the  boundaries  between  the  intrusive  and  the  parent  rock 
are  entirely  obliterated.  The  closely  related  aplites  are  also  usually 


THE  COMPOSITION  OF  IGNEOUS  ROCKS 


53 


older  than  the  lamprophyres  (Fig.  24),  although  the  contrary 
relationship  is  found  occasionally. 

Finally,  magmatic  differentiation  within  the  dike  itself  produces 
the  so-called  composite  or  double  dikes.  In  these  the  aplite  and 
lamprophyre  may  grade  into 
each  other  (Fig.  26),  or  they 
may  be  sharply  separated  (Fig. 
26).  Either  the  aplite  (Figs. 
25-26)  or  the  lamprophyre  (Fig. 
19)  may  form  the  selvedge. 

Petrographic  Provinces.— 
From  the  sequence  of  the  in- 
trusions and  extrusions  in  a 

Single     igneOUS     province,    there 

become  apparent  certain  laws 

which    can    be    explained    most 

11  .  .          bachkees,  near  the  Grossvenedieer. 

naturally    on    the    assumption 

that  closed  magma  basins,  in  which  differentiation  took  place, 
exist  within  the  earth.  The  rocks  of  such  a  region  (petrographic 
province)  ,  therefore,  have  a  certain  blood-relationship  (consan- 
guinity) which  shows  itself  in  the  constant  occurrence  or  predomi- 
nance of  certain  chemical  constituents. 


FIG.  24.—  A  system  of  approximately 


the  Central   granite   (Cr). 


[3 

Granite 

FIG.  25. 


EH 

Aplite 


Lamprophyre 

FIG.  26. 


FIGS.  25  AND  26. — Composite  or  double  dikes,  aplite  and  lamprophyre.    Fig.  25, 
showing  gradual  transitions;  Fig.  26,  with  sharp  contacts. 

It  is  very  difficult  to  explain  the  consanguinity  of  the  igneous 
rocks  of  one  region  and  then*  variation  from  those  of  an  adjacent 
region  if  all  the  phenomena  of  vulcanism  are  considered  the 
products  of  a  still-molten  earth-center.  And  the  assumption  that 
separate  and  relatively  independent  parts  were  produced  by  differ- 


54  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

entiation  from  a  very  heterogeneous  core,  and  that  successive 
extrusions  came  from  these  different  zones,  seems  rather  forced. 
It  is  much  more  likely  that  differentiation  takes  place  in  local 
magma  basins  which  are  detached  from  the  central  core. 

The  Christiania  basin  in  Norway,  which  is  one  of  the  most  complete  and  most 
carefully  studied  of  petrographic  provinces,  shows  a  definite  sequence  of  eruption 
in  a  sodium-rich  igneous  series.  The  oldest  formations  are  silica-  and  alkali-poor  dia- 
bases. These  are  followed  by  rocks,  richer  and  richer  in  soda  and  poorer  in  magnesia, 
iron,  and  lime,  until  nephelite-syenite,  with  11.5  per  cent,  of  soda,  is  reached.  In 
the  series  which  follows,  increasing  silica  is  accompanied  by  a  gradual  increase  in 
potash  at  the  expense  of  the  soda  until  true  granites,  with  2.5  per  cent,  of  soda,  7  per 
cent,  of  potash,  and  75  per  cent,  of  silica  are  reached.  The  series  is  closed  by  diabase, 
similar  to  that  first  erupted.  To  explain  these  occurrences  Brogger  assumed  the 
existence  of  a  peripheral  magma  basin  in  which  diffusion  led  to  extensive  differentiation. 
The  more  insoluble  constituents  concentrated  in  the  cooler,  upper  portion  of  the  basin, 
and  partially  solidified.  Later,  the  overlying  strata  sank  by  the  breaking  of  the  roof, 
forcing  the  upper  part  of  the  melt  outward  through  fisures.  In  a  similar  manner, 
subsequent  sinkings  of  the  crust  emptied  the  deeper  portions  of  the  basin  with  their 
progressively  more  soluble  constituents,  until  the  last  concentration  of  very  soluble 
materials  crystallized  as  granite.  A  final  sinking  brought  forth  the  last,  heavy,  basic 
remnant. 

Theories  of  Magmatic  Differentiation. — In  the  early  days  of 
petrography  much  stress  was  laid  upon  the  silica  contents  of  the 
different  rocks,  and  in  a  tabulation  of  results  all  possible  transitions 
from  persilicic  to  subsilicic  could  be  found.  Based  on  this 
gradation  was  the  oldest  theory  of  magmatic  differentiation  with 
a  scientific  basis,  namely  that  of  Bunsen,  which  postulated  the 
mixing  of  two  unlike  melts;  (I)  a  normal  trachytic,  and  (II)  a 
normal  pyroxenic,  with  the  following  compositions : 

I  II 

SiO2 76.67  48.24 

A12O3  +  Fe2O3 14.23  29.96 

CaO 1.44  11.57 

MgO 0.28          6.89 

K20 3.20  0.62 

Na20 4.18          1 . 92 

Bunsen  assumed  that  these  magmas  existed  in  separate  basins 
within  the  earth,  and  that  from  various  mixtures  of  the  two  all  of 
the  rocks  of  Iceland,  which  were  the  ones  he  examined  in  detail, 
could  be  produced.  Further  investigations  have  shown  that  the 
relationships  are  not  so  simple,  and  that  even  the  very  forced 
assumption  of  a  great  number  of  separate  magmas  will  not  suffice. 

Thus,  in  the  place  of  a  theory  of  mixed  magmas,  there  came  into 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  55 

existence  theories  of  magmatic  differentiation  in  which  the  magma 
was  regarded  either  as  a  mixed  salt-solution  or  as  a  mixture  of  fluids 
of  definite  composition.  If  the  magma  is  a  salt-solution,  the  rock- 
forming  minerals  were  dissolved  primarily  in  the  mineralizers,  and 
magmatic  differentiation  subsequently  concentrated  certain  con- 
stituents. The  differentiation  products,  therefore,  would  be  com- 
posed of  minerals  analogous  fco  those  of  the  parent  magma,  the  final 
differentiation  perhaps  resulting  in  eutectic  or  monomineral 
products. 

Rosenbusch's  "kern"  theory  is  a  somewhat  modified  form  of 
this  hypothesis,  the  more  important  of  the  kerns,  (Na,K)AlSiO4, 
CaAl2Si2O8,  RSiO3,  R2SiO4,  RA12O3,  and  SiO2,  having  the  composi- 
tion of  rock-forming  minerals.  Rosenbusch  assumed  that  the 
(Na,K)  AlSiO4  kern  occurs  nearly  pure  in  the  foyaites  (magma  <?) 
and  mixed  with  SiO2  in  granite  (magma  7).  The  RSiO3  and 
R2SiO4  kerns  appear  in  the  peridotites  (TT),  while  a  mixture  of  the 
(Na,K)  AlSiO4  and  CaAl2Si2O8  kerns  form  the  granodiorites  (5) 
and  the  gabbros  (i//) .  Finally,  a  combination  of  most  of  the  kerns 
forms  the  nephelite-rich  plagioclase  rock,  theralite  (0). 

The  converse  theories,  based  upon  the  mixing  of  different  melts  which  became 
separated  through  differentiation,  are  fundamentally  nothing  more  than  simple 
inversions  of  Bunsen's  theory. 

Piezocrystallization. — Besides  the  factors  already  considered 
there  are  certain  geologic  relationships  which,  in  some  cases,  have 
a  modifying  influence  upon  the  separation  of  the  minerals  from  a 
magma. 

The  granites  of  the  central  Alps  differ  decidedly  in  appearance 
from  normal  granites.  The  latter  are  granular  rocks  with  unoriented 
crystals  of  orthoclase,  plagioclase,  quartz,  and  mica.  The  typical 
granites  of  the  middle  Alps  have  a  similar,  haphazard-granular 
texture  in  their  central  portions,  but  toward  the  contact-zones 
this  gives  place,  more  and  more,  to  a  parallel-banded  texture,  and 
finally  to  true  schistosity.  The  mica  flakes,  which  were  unoriented 
in  the  center  of  the  mass,  here  lie  in  parallel  planes,  generally 
normal  to  the  direction  of  the  pressure  which  folded  the  mountains. 
Except  for  this  parallel  arrangement  of  the  mica,  the  two  kinds  of 
rock  are  exactly  alike.  All  of  them,  however,  differ  from  normal 
granites  in  many  ways,  both  megascopically  and  microscopically, 
yet  the  variations  cannot  possibly  be  secondary. 


56 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


For  example:  the  biotite,  which  is  absolutely  unaltered  by  weathering  on  account 
of  the  rapid  erosion  at  such  high  altitudes,  usually  occurs  in  parallel  intergrowths 
with  chlorite,  and  lies  in  sharp  contact  with  it.  In  appearance  it  differs  entirely  from, 
the  mica  of  ordinary  granite  which  has  become  cloudy  by  secondary  change  to  chlorite. 
Further,  the  plagioclase,  which  appears  perfectly  fresh  and  transparent  in  thin  sections, 
is  filled  with  sharply-formed,  unoriented  microlites  of  clinozoisite,  garnet,  muscovite, 
and  sillimanite,  in  some  cases  so  abundant  that  a  thick  section  of  the  mineral  appears 
dull  and  opaque.  Where  the  rock  has  a  distinct  parallel  texture,  these  minerals,  as 
well  as  the  mica,  occur  in  parallel  layers.  In  many  cases  the  feldspar  crystals,  plagio- 
clase as  well  as  orthoclase,  were  fractured  but  later  reunited  by  a  cement  of  quartz 
and  feldspar  upon  the  final  crystallization  of  the  mother-liquor.  Lastly,  the  quartz 
has  a  typical  cataclastic  texture,  indicating  the  stresses  through  which  it  passed. 
The  original  texture  was  naturally  made  indistinct  by  all  these  subsequent  changes, 
so  that  phenocrysts  of  feldspar,  where  found,  do  not  have  the  sharply-defined  outlines 
and  haphazard  arrangement  which  they  have  in  normal  porphyritic  granite,  but  lie 
with  their  flat  sides  parallel  to  the  schistosity  of  the  rock  and  occur  in  lenses,  usually 

fine-granular  at  the  borders.  On  account  of 
their  characteristic  appearance  in  cross-sections, 
these  rocks  are  called  Augen-gneisses  (Fig.  27). 
These  abnormal  characteristics  of  the  cen- 
tral Alpine  granites  were  considered  by  sup- 
porters of  the  theory  of  dynamometamorphism 
(Gr.  5vj>a/j,is,  force,  fjieTa/j.op<p6oiJiai,  to  be  trans- 
formed) to  have  been  produced  by  secondary 
causes,  such  as  orographic  pressures  and 
weathering,  in  rocks  which  originally  solidified 
as  normal  granites. 

It  was  formerly  supposed  that  pressure, 
without  the  aid  of  other  agencies,  was  sufficient 
to  cause  a  rearrangement  of  the  particles  in  the 
solid  state.  At  the  present  time  the  rearrange- 
ment is  generally  considered  dependent  upon 
the  latent  heat  and  water  content  of  the  rocks. 
The  fraction  of  1  per  cent,  of  water  present  in 


FIG.  27. — Porphyritic  Central 
granite  or  Augengneiss.  Scaradra- 
pass,  Graub linden. 


the  pores  is  supposed  to  act  as  a  concentrated  solvent,  dissolving  the  rock  consti- 
tuents, according  to  Riecke's  principle,  in  the  direction  of  the  pressure,  and  build- 
ing them  up  at  right  angles  to  this  direction,  thereby  producing  a  schistose  texture 
and  other  modifications.  A  thorough  rearrangement  of  the  solid  rock  without  the 
aid  of  a  dissolving  agent  is  less  plausible,  while  an  increase  in  temperature,  due 
to  erogenic  processes  alone,  sufficient  to  produce  even  partial  fusion,  is  still  less 
probable. 

The  central  granite  differs  from  normal  granites  in  mineral  composition  and 
texture.  It  contains,  namely,  hydroxyl-rich  chlorite,  and  there  are  great  quantities 
of  inclusions,  especially  in  the  plagioclases,  of  the  specifically  heavier  calcium-alumi- 
nium silicates,  such  as  garnet  and  clinozoisite.  The  plagioclases  themselves  are  poorer 
in  calcium-aluminium  silicates  than  are  those  in  normal  granites.  The  appearance  of 
the  biotite  and  chlorite,  as  already  mentioned,  indicates  their  contemporaneity  of 
formation,  while  the  garnet  and  clinozoisite  are  intergrown  with  the  fresh  feldspar  in 
a  manner  possible  only  for  primary  constituents.  The  chief  textural  difference  be- 
tween the  central  and  normal  granites  is  the  parallel  arrangement  of  the  mica  flakes 
in  the  former.  This  imparts  a  marked  schistose  texture,  especially  to  the  border- 
zone  of  the  massif.  Furthermore,  the  principal  constituents  are  broken  and  fractured 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  57 

in  the  schistose  as  well  as  in  the  non-schistose  rocks  of  the  entire  igneous  mass.  Since 
rocks  of  this  character  are  found  only  in  greatly  (Disturbed  regions,  the  attempt  to 
discover  a  connection  between  the  movement  and  the  peculiar  character  of  the  rock  is 
justified. 

The  question  arises :  How  does  a  cooling  magma  behave  under 
intense  strains  developed  during  its  solidification?  Orogenic 
pressure,  acting  in  a  definite  direction,  compresses  the  molten  mass, 
so  that  mica  flakes,  growing  in  the  border-zone  of  the  viscous 
magma,  will  develop  with  their  long  directions  at  right  angles  to 
the  pressure.  Pressure  in  the  melt,  however,  does  not  remain 
oriented  to  a  great  distance  but  soon  becomes  a  directionless  stress. 
At  some  distance  from  the  border,  therefore,  the  parallel  arrange- 
ment of  the  mica  flakes  is  lost,  and  the  rock  acquires  a  haphazard 
(richtungslose)  texture. 

The  great  pressure  upon  the  cooling  mass  tends,  under  the  given 
conditions,  to  produce  minerals  of  the  smallest  possible  molecular 
volumes.  In  spite  of  the  high  temperature,  a  part  of  the  water  with 
which  the  melt  is  saturated  goes  into  the  constitution  of  minerals 
which  would  not  be  stable  in  the  melt  under  normal  pressures, 
and  the  plagioclase  material  crystallizes  into  specifically  heavy  cal- 
cium-aluminium silicates  surrounded  by  a  border  of  calcium-poor 
feldspar.  In  this  way  there  is  gradually  formed  a  connected  frame- 
work whose  interstices  contain  the  still  fluid  mother-liquor  of  the 
granite.  Readjustments,  caused  by  the  pressure,  now  fracture  the 
brittle  constituents,  and  the  mother-liquor  is  forced  into  the  cracks 
thus  formed,  crystallizing  in  fine  veins  and  as  granular  quartz- 
feldspar  aggregates. 

If  large  tabular  crystals  of  feldspar  had  previously  separated 
from  the  melt,  these  arrange  themselves  in  the  border-zone  at  right 
angles  to  the  direction  of  pressure  and  become  enclosed  by 
parallel  layers  of  mica  flakes,  preventing  the  development  of 
crystallographic  boundaries  and  producing  lens-like  forms,  such 
as  are  characteristic  in  Augen-gneiss.  The  remainder  of  the  mother- 
liquor  finally  crystallizes  as  a  granular  aggregate.  If  schlieren 
occur  in  the  border-zones  of  such  a  molten  mass,  they  form  long 
drawn  out  layers  at  right  angles  to  the  direction  of  pressure,  but 
toward  the  center  of  the  mass  they  are  rounded. 

All  the  peculiarities  of  the  central  Alpine  granite  may  be  thus 
explained  on  the  assumption  of  powerful  lateral  pressure  operating 
during  the  crystallization  of  the  rock,  a  process  designated  by  the 


58 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


term  piezocry stabilization  (Gr.  Tuefo,  pressing  together).  Fig.  28 
is  a  section  showing  the  structural  alterations  from  edge  to  center 
in  an  elongated  central  Alpine  granite  massif  and  its  numerous 
apophyses  in  the  surrounding  schists.  In  the  intruded  mass  itself, 


Sedimentary  gneiss  Igneous  rock 

FIG.  28. — Cross-section  through  the  Central  Alpine  granite  massif  showing  apophyses 
resembling  interbedded  rocks. 

which  is  elongated  at  right  angles  to  the  direction  of  pressure,  the 
schistosity  runs  parallel  to  the  boundary.  In  the  dike-like  off- 
shoots, on  the  other  hand,  such  a  parallel  arrangement  cannot  be 
recognized  since  the  orientation  of  the  mica  plates  is  not  dependent 
upon  the  boundary  of  the  igneous  body  but 
upon  the  direction  of  the  pressure. 

A  definite  proof  that  the  parallel  structure 
of  the  central  Alpine  granite  is  not  secondary 
and  produced  by  dynamo-metamorphism  long 
after  the  solidification  of  the  rock,  is  afforded 
by  a  certain  boulder  from  the  Isar  (Fig.  29). 
This  consists  of  a  flat  fragment  of  an  aplite 
dike,  to  each  side  of  which  is  attached  a  piece 
of  schistose  granite.  The  smooth  surface  of 
the  aplite  shows  numerous  narrow  grooves 
into  which  biotite  plates  project  parallel  to 
the  schistosity  of  the  granite  and  perpendicu- 
lar to  the  contact  between  the  two  rocks. 
The  fragment  appears  to  be  part  of  the  walls 
of  a  fissure  into  which  the  fluid  aplitic  mass  flowed,  consequently 
the  granite  must  already  have  been  schistose  when  the  aplite, 
which  is  so  closely  related  to  it  genetically  and  in  time,  was  forced 
in.  The  sheets  of  aplite,  which  in  so  many  cases  are  present  in 


FIG.  29.— Boulder  from 
the  Isar,  near  Tolz. 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  59 

the  border-zones  and  parallel  to  the  schistosity  of  the  central 
Alpine  granite,  likewise  show  that  the  granite  was  schistose  when 
the  aplite  was  intruded. 

The  theory  of  piezocrystallization  presupposes  a  close  connection  between  a 
period  of  great  folding  of  the  Alps  and  the  intrusion  of  the  granite.  Nowadays  it  is 
customary  to  regard  mountain-formation  and  vulcanism  as  the  results  of  entirely 
different  processes,  yet  the  fact  that  most  folded  mountains  have,  granitic  cores  clearly 
shows  that  the  intrusions  are  connected  with  the  orogenesis.  On  the  other  hand,  a 
connection  between  intrusion  and  mountain-building  necessitates  a  relatively 
young  age  for  the  granite,  for  great  stresses,  such  as  those  required  for  piezocrystalliza- 
tion, were  probably  present  in  the  Alps  for  the  first  time  in  relatively  recent  periods. 

It  has  been  customary  to  regard  granites  in  general,  and  the  central  Alpine  granite 
in  particular,  as  very  old,  at  least  Paleozoic,  and  in  the  determination  of  the  age  of  the 
latter,  special  weight  has  been  attached,  for  two  reasons,  to  certain  characteristic 
basal  conglomerates  which  overlie  them:  (1)  because  the  petrographic  character  of  the 
previously  existing  rocks  may  be  determined  from  them;  and  (2)  because  the  less 
coarsely-grained  sediments  which  occur  in  this  region  are  few  and  usually  so  completely 
metamorphosed  that  their  original  character  can  be  determined  with  difficulty. 

Pebbles  of  granite,  undoubtedly,  are  present  in  the  conglomerates  which  directly 
overlie  the  main  granite,  for  example  the  Rannach  conglomerate  of  the  Low  Tauern, 
but  the  pebbles  are  of  normal  granite  and  differ  entirely  from  the  rock  in  place.  To 
establish  a  connection  between  these  two  granites  it  has  been  assumed  that  the  central 
massif  was  recrystallized  by  dynamometamorphism,  while  the  boulders,  embedded  in 
a  softer  matrix,  retained  their  original  condition,  but  there  is  no  justification  whatever 
for  such  an  assumption.  On  the  contrary,  the  evidence  against  it  is  strong,  for  the 
innumerable  aplitic  beds  of  so-called  micro-tourmaline-gneiss,  which  occur  in  the  strata 
overlying  the  conglomerate,  can  be  regarded  only  as  apophyses  of  the  undoubtedly 
younger  granite.  Furthermore,  the  cementing  material  of  the  conglomerate  itself  is 
thoroughly  recrystallized  and  impregnated  with  tourmaline  by  cohtact-metamor- 
phism. 

The  anomalous  character  of  the  central  Alpine  granite  is  un- 
questionably related  to  the  mountain-building.  Were  the  granite 
originally  normal  and  only  subsequently  altered  by  dynamometa- 
morphism, it  might  be  of  any  age,  and  the  metamorphism  might 
have  been  produced  at  any  period  of  folding  and  be  entirely  inde- 
pendent of  the  intrusions.  If,  on  the  other  hand,  its  anomalous 
character  is  original,  then  the  granitic  intrusion  and  the  mountain- 
folding  were  closely  connected  chronologically. 

There  are  many  field  observations  which  force  one  irresistibly 
toward  the  hypothesis  of  piezocrystallization.  Beside  the  relation 
of  the  aplite  to  the  granite,  shown  in  Fig.  29,  and  the  fact  already 
mentioned  that  aplite  dikes  occur  parallel  to  the  schistosity,  there 
is  present  in  many  cases  a  noteworthy  difference  between  these 
two  rocks.  It  is  true  that  aplites  and  pegmatites  in  many  parts 
of  the  central  Alps  are  just  as  distinctly  schistose,  and  in  their 


60  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

interiors  just  as  much  brecciated  by  orogenic  movements  as  the 
central  granite  itself,  but  elsewhere,  as  in  the  Mont  Blanc  region, 
the  effects  of  piezocrystallization  as  well  as  of  cataclastic  action 
are  less,  even  in  the  youngest  granites.  In  the  greater  part  of  the 
Alpine  region  the  aplites  differ  from  the  central  granite  in  having 
been  subjected  to  no  mechanical  stresses,  and  in  having  remained 
uniform  rocks  without  schistosity,  showing  neither  mineralogical 
nor  structural  variations  from  their  normal  habit. 

The  above  argument  may  not  be  convincing  to  those  who  do 
not  believe  in  a  close  time-connection  between  the  intrusion  of  the 
aplite  and  the  granite  itself,  but  who  think  that  these  rocks  origi- 
nated, perhaps  like  ore-veins,  from  aqueous  solutions  within  the 
previously  cooled  igneous  rock.  Such  views,  however,  cannot  be 
held  by  anyone  who  has  studied  the  petrographic  relationships. 
The  consanguinity  of  granite  and  aplite  is  unmistakable,  and  all 
observations  show  clearly  that  aplite  is  crystalline-granular  only 
where  it  was  forced  into  granite  which  was  still  hot  and  in  the  last 
stages  of  solidification. 

That  the  orogenic  processes,  at  least  in  the  main,  ceased  with  the  crystallization 
of  the  granite,  is  shown  further  by  the  fact  that  in  the  schist-zone  surrounding  the 
central  granite  of  the  Grossvenediger,  in  the  High  Tauern,  almost  all  traces  of  cata- 
clastic action  are  wanting,  even  in  the  very  sensitive  quartz-rich  rocks.  This  phe- 
nomenon cannot  be  explained  on  the  theory  of  dynamometamorphism,  for  according  to 
this  the  surrounding  schists  especially  should  owe  their  crystallinity  to  orogenic  move- 
ments, and  consequently  should  show  evidences  of  its  action.  Furthermore,  accord- 
ing to  this  theory,  the  same  processes  as  those  which  produced  a  cataclastic,  schistose 
rock  from  the  originally  massive,  unfractured  central  granite,  must  have  altered  vari- 
ous argillites,  limestones,  sandstones,  and  effusive  rocks  to  crystalline  schists  without 
leaving,  in  the  structures,  any  evidence  of  the  mechanical  stresses  which  metamor- 
phosed them. 

A  consideration  of  these  relationships  shows,  without  question,  that  neither  the 
youngest  granite  formation  of  this  district,  nor  the  satellite  aplites,  nor  the  metamor- 
phosed country-rocks,  suffered  any  considerable  orogenic  movement  since  arriving  in 
the  crystalline  state,  consequently  between  the  intrusion  of  the  principal  granitic  mass 
and  the  development  of  its  anomalous  character,  no  very  great  time  had  elapsed; 
time,  that  is,  as  measured  by  geologic  periods. 

The  central  granite,  therefore,  represents  a  comparatively  young  formation,  whose 
intrusion  doubtless  was  closely  associated  with  the  process  of  mountain-building. 
Although  the  rocks  of  the  earth's  crust  were  compressed  by  the  folding,  it  is  extremely 
probable  that  at  the  same  time  some  of  the  molten  material  was  squeezed  out  through 
the  fractured  crust.  The  enormous  overlying  load,  which  was  greatly  increased  by 
the  folding  of  the  blocks,  and  the  activity  of  the  magma  caused  the  latter  to  ascend 
through  fissures  in  the  bedded  rocks,  and,  partly  of  its  own  accord,  partly  pushed,  the 
molten  mass  forced  itself  into  the  weaker  parts  of  the  bedded  rocks.  It  must  not  be 
assumed  that,  under  such  enormous  pressures,  there  were  empty  spaces  into  which  the 
rock-material  could  be  poured.  The  magma  first  had  to  provide  itself  with  room  by 


THE  COMPOSITION  OF  IGNEOUS  ROCKS 


61 


forcing  apart,  tearing,  and  raising  the  bedded  rocks.  Thus,  in  the  course  of  a  long 
period  of  time,  the  mighty  masses  which  form  the  axis  of  the  Alps  gradually  arose. 
The  so-called  fan-structure  of  the  Alps  doubtless  arose  from  this  slow  squeezing 
together  of  the  rocks.  The  magma  rising  from  below  was  pinched  in  at  the  base,  where 
the  strata,  on  account  of  the  pressure,  could  not  be  shoved  aside.  The  principal  mass, 
consequently,  was  forced  out  above.  This  is  shown  in  Figs.  30  and  31,  which  approxi- 
mately represent  the  fans  of  Mont  Blanc  in  two  stages  of  their  development. 


t  t 

FIG.  30.  FIG.  31. 

FIGS.  30  AND  31. — Origin  of  the  fan  structure  through  the  intrusion  of  a  magma 
during  mountain  folding. 

Injection  of  the  Adjacent  Rocks. — The  extensive  mechanical 
results  produced  by  the  intruding  mass  upon  the  adjacent  rocks 
clearly  shows  that  the  molten  magma  was  not  a  viscous  mass  which 
was  later  passively  pushed  by  tectonic  processes  to  the  place  of  its 
solidification.  Contact-breccias  (Fig.  32)  and  much  foldhig  of 
the  country-rock  near  the  contact  and  a  gradation  into  fine  fold- 


FIG.  32. — Contact  breccia.     Geyer,  Erzgebirge. 

ing  away  from  it,  show  the  activity  of  the  magma.  The  intrusion 
of  the  magma,  and  especially  the  intrusion  of  its  much  more  mobile 
aplitic  differentiation  products,  into  the  finest  fracture  and  schis- 
tosity  planes  of  the  adjacent  rock,  and  the  exfoliation  of  the 
schistose  border  and  its  complete  saturation  with  molten  material 


62  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

(Fig.  33),  that  is,  its  injection,  show  this  active  property  especially 
well. 

The  melt  itself,  saturated  with  mineralizers,  may  have  been  ex- 
plosive under  certain  conditions.  The  belief  that  intrusive  masses 
fill  former  cavities  in  the  earth's  crust,  or  that  the  igneous  material 
of  dikes  was  poured  into  gaping  fissures,  is  entirely  erroneous. 
The  intrusion  of  molten  masses,  in  the  majority  of  cases,  un- 
doubtedly had  a  certain  connection  with  orogenic  processes.  This 
may  be  seen,  for  example,  in  the  arrangement  of  a  series  of  granite 
stocks  parallel  to  the  chief  lines  of  weakness  of  a  region,  or  the  in- 
trusion of  large  granite  masses  parallel  to  the  axis  of  a  folded  moun- 
tain range,  yet  in  neither  case  is  it  probable  that  the  magma  flowed 
into  open  cavities.  It, is  much  more  likely  that  the  melt,  over- 
saturated  with  gases  and  vapors,  broke  through  places  most 


FIG.  33. — Very  thin-bedded  and  folded  injection  schist.     Oberpfalz. 

weakened  by  tectonic  processes  and,  rupturing  the  crust,  lifted  the 
upper  strata  to  enormous  heights.  The  folding  of  rocks  already 
schistose  caused  a  loosening  of  the  lamellae,  and  between  these  the 
volcanic  material  forced  itself,  principally  parallel  to  the  schis- 
tosity.  This  intrusion  caused  such  an  extensive  tearing  apart  of 
the  schist  that  shreds  and  fragments  of  it  are  enclosed  in  the  solidi- 
fied igneous  rock.  Where  the  magma  forced  itself  in  the  form 
of  a  laccolith  between  approximately  horizontal  beds,  the  over- 
lying strata  were  first  attacked,  and  into  the  spaces  between  its 
exfoliating  plates,  the  molten  rock  entered.  Where  the  beds  were 
steeply  inclined,  they  were  torn  apart  for  great  distances  along  the 
bedding,  so  that  a  rock  with  alternating  parallel  bands  of  granite 
and  schist  resulted.  In  less  schistose  rocks,  such  as  limestones, 
sandstones,  etc.,  or  where  fractures  extended  to  considerable 
depths  through  the  sediments,  the  intrusive  masses  forced  their 


THE  COMPOSITION  OF  IGNEOUS  ROCKS 


63 


way  across  the  schist osity  along  fissures  and  fractures,  widening 
them  locally  to  true  stocks. 

The  country-rocks  of  all  stock-like  masses  are  not  injected  to 
the  same  degree  because  the  action  is  much  less  marked  where 
the  invaded  rocks  are  less  schistose,  or  at  least  less  exfoliated. 
But  even  here  the  magma  was  forcibly  intruded,  as  is  shown  not 
only  by  the  wide  distribution  and  partial  assimilation  of  fragments 


FIG.  34. — Hornfels  with  parallel  bands  of  granite.     Riesenberg,  near  Ossegg,  Bohemia. 

of  the  wall-rock  in  the  intrusive,  but  also  by  the  apophyses  which 
in  many  cases  extend  into  the  surrounding  rocks  to  great  distances. 
Where  hornfels  is  banded  in  this  manner  by  granite  and  aplite 
(Fig.  34),  it  may  be  very  similar  in  appearance  to  an  injection- 
schist.  In  other  places  the  apophyses  cut  the  country-rock  in 
entangled  veins,  or  follow  particularly  prominent  planes  of  schis- 
tosity  (Fig.  35). 


FIG.  35. — Hornfels  torn  apart  along  the  bedding  by  granite.     Gefrees,  Fichtelgebirge. 

The  effect  of  the  intrusive  upon  the  country-rock  depends  more 
upon  the  latter  than  upon  the  former.  For  example,  the  aplites 
of  the  central  Alpine  schist-zone  lose  their  typical  aplitic  characters 
where  they  invade  granular  limestones. 

Where  the  schistose  structure  was  well  developed  in  the  rocks,  or  where  joints  had 
previously  been  started  by  erogenic  processes,  the  molten  magma  was  usually  injected 
into  the  schists  to  great  distances.  This  is  shown  in  Fig.  36,  which  represents  a  granite 


64 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


mass  with  included  fragments  of  greatly  crumpled  schist,  the  latter  filled  with  aplitic 
material  even  in  its  finest  cracks.  Where  such  blocks  occur  in  the  border-zone  they 
form  apparently  homogeneous  beds  which,  on  superficial  examination,  give  the  im- 
pression of  schistose  or  banded  rocks  (Fig.  37).  They  have,  consequently,  been  called 
gneisses,  or,  more  specifically,  banded-  or  vein-gneisses.  The  granite  has  had  a  pow- 


Schist  Granite 

FIG.  36. — Granite  injections  in  the  country-rock. 

erful  dissolving  effect  upon  the  schist  on  account  of  the  intimate  contact  between  the 
two,  and  numerous  accessory  minerals,  such  as  cordierite,  iron-garnet,  aluminium  sili- 
cates, etc.,  whose  constituents  were  taken  from  the  country-rock,  occur  widely  dis- 
tributed in  the  injected  dikes.  The  intrusive  rock  itself  may  not  be  exposed  in  every 
case,  but  the  injected  material  and  the  partial  resorption  of  the  schists  prove  that 


FIG.  37. — Schieferhornfels  banded  by  granitic  injections. 

Klemm,  photo.) 


Odenwald.     (Prof.   Dr. 


an  igneous  body  must  be  present  at  no  great  depth.  In  a  number  of  such  cases,  as  in 
the  Hohen  Venn  and  in  the  Henry  Mountains  in  Utah,  great  masses  of  granite  which 
did  not  appear  at  the  surface  have  actually  been  exposed  at  a  later  time  by  the  digging 
of  tunnels,  shafts,  etc. 

A  careful  examination  of  contact-breccias,  irregular  apophyses 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  65 

in  hornfels,  injection-schists,  and  partially  or  entirely  resorbed 
schists,  shows  that  they  are  entirely  comparable  formations.  It 
is  true  that  they  appear  quite  different  externally,  yet  they  all 
originated  by  the  same  igneous  processes. 

It  was  long  before  the  metamorphism  of  the  so-called  Archean 
formation  was  recognized  as  being  due  to  contact-metamorphism 
or  to  injections  and  resorption  by  granitic  intrusions.  The 
effects  of  well-characterized,  chemico-geological  processes  which 
are  here  apparent  had  wrongly  been  considered  indicators  of  a 
definite  geologic  age,  especially  by  French  geologists.  Recently, 
however,  they  recognized  that  in  many  of  these  gneisses  the 
boundaries  between  the  injected  granite  and  the  country-rock 
become  gradually  more  and  more  indistinct  until  the  rock  presents 
a  nebulous  appearance  (Nebulite,  Fig.  38),  while  other  extensive 


FIG.  38. — Nebulite,  or  schist  resorbed  by  granite.     Vilschofen,  near  Passau. 

areas  of  such  injection-rocks  occur  without  the  immediate  presence 
of  any  igneous  mass.  This  led  to  a  new  theory  directly  opposed 
to  the  former. 

According  to  the  latter  view,  the  materials  of  the  crystalline 
schists  were  brought  under  such  conditions  by  orogenic  processes 
that  gases  and  vapors,  either  from  the  molten  interior  itself  or 
from  a  peripheral  magma  basin,  entered  into  them  and  produced 
molecular  rearrangements,  after  the  manner  of  the  agents  of 
contact-metamorphism.  The  action,  however,  was  not  limited  to 
the  production  of  new  constituents  from  the  materials  of  the  old. 
In  the  mineralizer-saturated  mass,  far-reaching  diffusion  processes 
took  place  and  gradually  led  to  a  kind  of  liquation,  so  that,  from 
the  originally  homogeneous  magma,  two  partial  magmas  were 


66  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

produced,  resulting,  for  example,  in  the  production  of  alternating 
light  and  dark  bands  in  the  so-called  banded-  or  vein-gneisses. 

This  theory  may  be  regarded  as  a  reconstruction  of  Button's  view  that  regional 
metamorphism  is  caused  by  subterranean  heat,  a  theory  seemingly  substantiated  by 
recent  observations  on  the  universal  distribution  of  tourmaline  and  the  mineralizing 
action  of  superheated  gases.  This  process  has  been  called  pneumatolitic-met&moT- 
phism,  but  this  is  rather  a  makeshift  term  used  to  explain  occurrences  in  which 
the  active  agent  of  normal  contact-metamorphism,  namely  the  granitic  intrusive  rock, 
either  is  not  exposed  or  its  connection  with  the  metamorphism  is  not  recognized. 
Petrographically  there  are  no  grounds  for  introducing  a  second  group  of  rocks,  iden- 
tical with  that  which  was  produced  by  contact-metamorphism. 

Type -mixing. — The  most  common  kinds  of  rock  mixtures  are 
the  injection-rocks  and  those  produced  by  the  assimilation  of  older 
rocks  by  igneous  magmas.  Such  mixtures  are  most  characteristic, 
as  well  as  most  common,  in  granites,  although  they  are  not  wanting 
among  other  deep-seated  rocks.  The  solution  of  rocks  which  are 
entirely  different  chemically  disturbs  the  equilibrium  of  the  magma, 
and  diffusion  phenomena  appear,  leading  especially  to  the  separa- 
tion of  schlieren,  as  was  described  in  detail  in  the  chapter  on 
magmatic  differentiation.  Many  mixed-types  simply  represent 
separations  produced  by  a  saturation  by  mineralizers,  as  was  shown 
at  the  conclusion  of  the  last  chapter.. 

The  dissolving  power  of  mineralizer-rich  magmas  is  exemplified 
in  pegmatites,  which,  in  many  cases,  entirely  change  their  char- 
acter upon  passing  into  chemically  different  rocks,  so  that  only  by 
their  geologic  continuity  can  the  different  portions  be  shown  to  be 
parts  of  the  same  intrusion.  In  many  places  the  type-mixing  has 
gone  so  far  that  hardly  a  trace  of  the  normal  pegmatite-minerals 
remains.  The  first  stage  in  the  alteration  is  shown  by  the  appear- 
ance of  occasional  crystals  of  garnet,  cordierite,  epidote,  spinel, 
etc. ;  the  final  stage  in  many  cases  by  the  production  of  anomalous 
orthoclase-  and  quartz-free  rocks,  such  as  are  abundant  in  so-called 
gneisses.  Other  examples  were  given  on  page  52. 

The  more  slowly  such  a  mixed-rock  solidifies,  the  more  com- 
pletely are  the  foreign  constituents  dissolved,  while  at  the  same 
time  the  non-uniform  condition  and  parallel  structure,  which 
were  brought  about  by  diffusion,  become  more  and  more  indis- 
tinct on  account  of  the  protracted  crystallization.  The  resulting 
rock,  therefore,  may  be  quite  uniform  in  appearance.  The  country- 
rock  itself  is  changed  by  the  addition  of  foreign  constituents 
through  the  agency  of  mineralizers  from  the  adjacent  intrusive. 


THE  COMPOSITION  OF  IGNEOUS  ROCKS 


67 


This  is  shown  by  the  occurrence  of  tourmaline  in  most  rocks  that 
have  been  altered  by  contact-metamorphism.  In  many  places 
there  has  been  an  addition  of  great  quantities  of  new  constituents, 
which,  in  the  case  of  schistose  rocks,  became  uniformly  distributed 
and  produced  an  unquestionable  mixed-type.  Feldspar,  especially 
albite,  is  one  of  the  most  common  of  the  introduced  minerals. 
It  occurs  in  certain  hornstone-like  adinoles,  and  is  especially 
abundant  and  extends  to  great  distances  from  the  intrusive  rock 
in  albite-gneiss  or  schistes  feldspatises. 

In  the  selvedges  of  dikes,  and  adjacent  to  inclusions  in  hypabyssal  and  extrusive 
rocks,  mixed-types  are  widely  distributed,  although  they  are  usually  only  of  local 
significance.  They  are  very  common  in  silica-poor  igneous  rocks,  especially  in  the 
lamprophyres.  As  examples  may  be  mentioned  quartz  and  feldspar  lenticles  and 
granite  shreds  in  granitic  lamprophyres, 
and  olivine  nodules  in  basalts.  The 
latter,  which  probably  represent  por- 
tions of  the  magma  which  crystallized 
deep  within  the  earth,  produced  con- 
siderable alteration  in  the  rock-type 
when  they  were  carried  upward  in  great 
amounts  by  the  rising  magma.  Silica- 
and  alumina-rich  inclusions  are  usually 
surrounded  by  resorption-rims  which 
contain  more  glass  than  the  normal 
rock.  In  some  cases  silica-rich  inclu- 
sions in  basalts  and  in  other  basic  ig- 
neous rocks  have  been  entirely  assimi- 
lated, their  former  presence  now  being 
indicated  only  by  the  occurrence  of  Quartz-porphyry  Trap 

glass-rich   schlieren.     Finally,   the  sel-       FlG'  39,-Type-mixing.     Bennan  Head, 

.  J.S16  oi  *"i.rrn.n. 

vedges     of     lamprophync     dikes     are 

nearly  or  entirely  of  glass  at  the  contact  with  silica-rich  rocks,  for  example  sordawaldite 
and  wichtisite  in  granite,  but  such  glassy  zones  are  abnormal  in  ordinary  lamprophyres. 

The  relationships  where  a  silicic  and  a  basic  magma  were  mixed  while  still  in  the 
fluid  condition  are  of  especial  interest.  For  example,  at  Bennan  Head,  on  the  Island 
of  Arran,  a  sheet  of  quartz-porphyry,  with  large  quartz  and  feldspar  phenocrysts, 
lies  above  one  of  diabase  (Fig.  39).  In  the  latter,  and  especially  in  the  lower  part  of 
the  sheet,  the  same  minerals  occur  as  phenocrysts.  Between  the  two  extruslves 
there  is  a  considerable  zone  of  intermediate,  light  gray  rock,  consisting  microscopically 
of  an  intimate  mixture  of  the  micropegmatitic  groundmass  of  the  quartz-porphyry 
with  the  ophitic  trap.  In  spite  of  their  perfect  mixing,  the  two  kinds  of  rock  have 
preserved  their  individuality.  In  all  probability  the  quartz-porphyry  wras  poured  out 
over  the  still  molten  diabase,  so  that  the  two  magmas  mingled  at  the  contact,  and 
numerous  phenocrysts  of  quartz  and  feldspar  from  the  quartz-porphyry  sank  into  the 
underlying  diabase.  This  mixture,  however,  of  "normal  trachytic"  and  "normal 
pyroxenic"  magmas,  in  the  sense  of  Bunsen,  did  not  produce  an  intermediate  type,  but 
produced  an  extremely  anomalous  mixed-rock.  The  occurrence,  therefore,  is  excellent 
evidence  against  Bunsen's  mixture  theory. 

Szabo  used  the  term  mixed-type  in  a  different  sense  from  that  used  above.     He 


68  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

applied  it  to  normal,  intermediate  igneous  rocks;  so  far,  at  least,  going  back  to  Bunsen's 
theory. 

Graphical  Representations  of  Chemical  Compositions. — Numerous  attempts  have 
been  made  to  represent,  in  a  simple  way,  either  graphically  or  by  formulae,  the  com- 
positions of  igneous  rocks.  The  complex  nature  of  the  bisilicates,  however,  prevents 
a  perfectly  clear  representation. 

The  formulae  of  Michel-Levy,  which  represent  textures  and  mineral  compositions 
at  the  same  time,  have  proved  to  be  much  too  complicated  for  general  use.  His  sym- 
bols are  as  follows : 

T  granular,  n  porphyritic,  a  granitic,  0  granulitic,  7  micropegmatitic,  5  gabbroic, 
w  ophitic.  For  porphyritic  rocks,  <p  spherulitic  to  microfelsitic,  ir  glassy  with  traces  of 
devitrification,  n  microlitic  with  glassy  groundmass. 

The  dark  constituents  are  designated  by  capital  letters;  FI  magnetite,  F2  titanite, 
Fz  spinel,  F4  perofskite,  F5  apatite,  FG  zircon,  F7  titanite,  F8  orthite,  F9  garnet, 
0  olivine,  Hi  hypersthene,  £T2  bronzite,  H3  enstatite,  PI  aegirite,  P2  malacolite, 
P3  diallage,  P4  augite,  AI  soda  amphibole,  A2  green  amphibole,  As  brown  amphibole, 
M  biotite,  and  the  light-colored  constituents  by  small  letters,  I  leucite,  n  nephelite, 
h  melilite,  «i  sodalite,  s2  hauynite,  s3  noselite,  ai  orthoclase,  a\  microcline,  a2  anortho- 
clase,  «s  albite,  t  soda-lime  feldspars,  t\  oligoclase-andesine,  t%  labradorite,  fa  anorthite, 
q  quartz,  m  muscovite. 

In  the  formulae  the  Greek  letter  which  represents  the  texture  is  written  first,  then, 
after  a  hyphen,  follow  the  constituents  in  the  order  of  their  crystallization,  those 
of  the  first  period  of  consolidation  being  marked  by  a  vinculum,  and  those  of  the 
second  by  underscoring,  as  shown  in  the  following  example: 


Tco  -  (F1.2.3)OP4A3M(t2.3)(P3.4). 

This  represents  a  rock  whose  texture  is  granular-ophitic.  The  minerals  of  the 
first  generation  are  magnetite,  titanite,  apatite,  olivine,  augite,  brown  hornblende, 
and  biotite,  while  those  of  the  second  are  labradorite  to  anorthite,  diallage,  and  augite. 
The  rock  is  a  basalt.1 

Chemical  rock-formulas  are  usually  based  upon  molecular  proportions,  and  are 
comprehensive  and  concise.  That  of  Osann  is  the  one  most  commonly  used,  and  rules 
for  recalculating  analyses  according  to  his  system  are  here  given.2  This  classification 
is  not  intended  as  a  substitute  for  the  textural  and  mineralogical  classification  so 
long  in  use,  but  is  intended  to  supplement  it  and  make  it  more  complete. 

1.  Add  small  amounts  of  Cr2O3  to  the  A12O3. 

2.  Determine  whether  the  percentage  of  A12O3  is  greater  or  less  than  the  sum  of  the 
alkalies. 

If  A12O3  is  less  than  the  sum  of  the  alkalies,  reduce  all  Fe2O3  to  FeO  by  multiply- 
ing its  percentage  weight  by  0.9. 

1  CHRUSTSCHOPF  proposed  a  modification   of   MICHEL-LEVY'S   formulae    (Neues 
Jahrb.,  1891,  II,  225).     J. 

2  The  rules  for  OSANN'S  classification  given  in  this  translation  are  considerably 
more  detailed  than  in  WEINSCHENK'S  original.     The  original  articles  by  OSANN  are 
in  Tschermak's  Mitteilungen  XIX  (1899-1900),  351-469;  XX  (1901),  399-558;  XXI 
(1902),  365-448;  XXII  (1903),  322-356,  403-436.     See  further  F.  BECKE,  Tschermak's 
Mitteilungen  XXII  (1903),  214;  E.  KAISER,  Centralbl.  /.  Min.,  etc.,  1904,  338.    OSANN'S 
classification  is  to  be  revised  and  improved  in  the  near  future.     In  litteris  Osann, 
April  11,  1915.     /. 


THE  COMPOSITION  OF  IGNEOUS  ROCKS  69 

If  A12O3  is  greater  than  the  sum  of  the  alkalies,  it  is  taken  care  of  in  rule  7,  etc., 
below. 

3.  Recalculate  the  analysis  to  molecular  proportions,  omitting  H2O,  C02,  F,  and  S. 

4.  Recalculate  these  molecular  proportions  to  100. 

5.  Let  s  be  the  number  of  molecules  of  SiO2,  TiO2,  and  ZrO2. 

s  =  SiO2  +  TiO2  +  ZrO2 

6.  A12O3>(K,  Li)2O  +  Na2O. 

(a)  After  adding  Li  to  K,  allot  A12O3  to  all  the  (Li,  K)2O  and  all  the  Na2O  in  the 
proportions  of  one  to  one  to  form  (K,  Li)2O-Al2O3  and  Na2O-Al2O3.  Let  A  represent 
the  number  of  molecules  so  formed,  that  is 

A  =  (K,  Li)20  +  Na20. 

(6)  Allotment  of  remaining  A12O3. 

If  A12O3>(K,  Li)2O  -+  Na2O  but  less  than  CaO  +  (K,  Li)2O  +  Na2O,  the  re- 
maining A12O3,  after  6a  is  satisfied,  is  united  in  equal  proportions  with  CaO  to  form 
CaO-Al2O3.  The  number  of  molecules  thus  formed  is  represented  by  C.  That  is, 

C  =  A1203  -  A. 

(c)  If  Al2O3>CaO  +  (K,  Li)2O  +  Na2O,  add  BaO  and  SrO  to  CaO  and  unite  in 
equal  proportions  with  A12O3.  If  any  A12O3  still  remains  unused,  take  enough 
(Mg,  Fe)O  to  satisfy  it,  and  add  this  to  C  as  a  molecule  of  (Mg,  Fe)O-Al2O3. 

7.  A12O3<  (K,  Li)2O  +  Na2O. 

(a)  Take  an  amount  of  Fe2O3  equal  to  the  remaining  alkali  (amount  of  excess  of 
alkalies  over  A12O3)  to  form  the  molecule  (K,  Li)2O-Fe2O3,  and  add  this,  as  aegirite 
molecules,  to  A.  The  numerical  value  of  A  still  remains 

A  =  (K,  Li)2O  +  Na2O. 
(6)  Excess  of  alkalies. 

n  A12O3  +  Fe2O3<(K,  Li)2O  +  Na2O,  the  excess  of  alkalies  is  to  be  added  to  A 
as  a  separate  group,  so  that  A  contains  the  total  alkalies  as  before. 

8.  The  sum  of  all  the  molecules  of  MnO  and  NiO,  and  of  the  molecules  of  CaO 
not  used  in  rule  66,  and  the  molecules  of  FeO,  MgO,  SrO,  and  BaO  not  used  in  rule  6c, 
is  represented  by  F.     That  is, 

F  =  (CaO  +  FeO  +  MnO  +  MgO  +  NiO  +  SrO  +  BaO)  -  C. 
The  Fe2O3  not  used  in  rule  7  is  recalculated  as  FeO  and  is  added  here. 

9.  Recalculate  Na2O  plus  (K,  Li)2O  to  10,  and  indicate  the  value  of  Na2O  thus 
obtained  by  n.     That  is, 

10Na2O  10Na2O 


n  = 


(K,  Li)20  +  Na20 


10.  Recalculate  the  sum  of  A,  C,  and  F  to  20,  and  represent  these  proportions, 
to  the  nearest  0.5,  by  o,  c,  and/. 

a  +  c  +  /  =  20. 

11.  The  rock-formula  now  may  be  written  in  terms  of  s,  a,  c,  /,  and  n,  as  shown 
below.     It  may  also  be  plotted  on  a  triangular  diagram  whose  vertices  are  designated 
by  20a,  20c,  and  20/.     The  rock  is  indicated  by  the  point  where  the  three  lines,  which 
represent  these  values,  cross  (Fig.  43). 

Two  other  values  are  used  occasionally  in  certain  rocks,  but  they  are  not  so 
important  as  s,  A  (a),  C(c),  F(f),  and  n.     They  are  as  follows: 

12.  Recompute  (Mg,  Fe,  Mn,  Ni)O  and  (Ca,  Ba,  Sr)O  as  used  in  F,  so  that  their 


70  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

sum  equals  10,  and  let  m  represent  the  value  of  (Mg,  Fe,  Mn,  Ni)O  thus  obtained. 
That  is, 

_  10(Mg,  Fe,  Mn,  Ni)O 

=   (Mg,  Fe,  Mn  Ni)O  +  (Ca,  Ba,  Sr)O ' 

The  silica  coefficient  (/c)  has  the  following  value: 


k  = 


6A+2C  +F 


As  a  check  on  the  work  we  have  s  +  2A  +  2C  +  F  =  100. 

For  rapid  calculation,  Kaiser  used  the  following  additional  formulae.     After  com- 
puting the  molecular  proportions  and  reducing  to  100,  he  determined 

N  =  100  -  (s  +  A12O3  +  P2O5), 

p  =  4 

A1203 
V-      N    ' 

From  which 

a  =  20P, 

c  =  20(Q  -  P), 

/  =  20(1  -  Q). 

These  formulae,  with  the  essential  formulae  of  Osann, 

s  =  SiO2  +  ZrO2  +  TiO2, 
A  =  (K,  Li)2O  +  Na2O, 
10Na2O 
A       ' 


n  = 


are  all  that  are  necessary  for  a  computation. 

In  the  above  formulae  A  (a)  represents  fairly  accurately  the  amount  of  alkali- 
feldspar  and  feldspathoids  in  the  rock;  nephelite,  leucite,  sodalite,  hauynite,  the 
segirite  molecule,  the  potash  molecule  in  mica,  and  the  alkali  molecule  of  aenigmatite 
and  eucolite  here  being  considered  feldspathoids.  No  definite  amount  of  SiOa  is 
added  to  A  to  form  these  minerals  from  the  alkalies,  and  they  do  not  necessarily  occur 
as  such  in  the  rock.  To  determine  the  minerals  accurately  one  must  know  the  relative 
amounts  of  the  individual  constituents  and  their  actual  compositions.  The  variable 
amounts  of  alkali  in  the  micas  and  in  the  feldspathoids  make  the  conversion  difficult. 
In  the  case  where  Al2Oa<(K,  Li)2O  +  Na20,  part  of  the  alkali  unites  with  iron  oxide 
to  form  alkali-bearing  pyroxenes  or  amphiboles.  This  is  usually  the  case  only  in  rocks 
which  are  alkali-rich  and  which  contain  feldspathoids.  In  these  rocks,  naturally,  C 
equals  zero,  and  the  projection  points  lie  on  the  A-F  line  of  the  diagram. 

C(c)  represents  the  amount  of  the  anorthite  molecule  in  the  plagioclase.  In 
subordinate  amounts  the  aluminium-bearing  molecules  of  pyroxene  and  amphibole 
are  contained  here,  and,  in  melilite  rocks,  the  gehlenite  molecule  also. 

F(f)  represents  the  relative  amounts  of  the  dark  constituents.  It  contains  the 
AUOs-free  and  the  alkali-free  pyroxene  and  amphibole  molecules,  the  olivine  molecule 
in  olivine  and  mica,  the  akermanite  molecule  in  melilite,  the  iron  content  of  the  iron 
ores,  and  the  lime  content  of  apatite  and  titanite. 

n  gives  broadly  the  ratio  of  the  orthoclase  to  the  albite,  or  of  the  potash  feld- 
spathoid  to  the  soda-feldspathoid. 

k  is  the  silica  coefficient,  and  shows  the  ratio  of  SiO2  to  A,  C,  and  F.     To  obtain 


THE  COMPOSITION  OF  IGNEOUS  ROCKS 


71 


k,  therefore,  the  value  s  must  be  divided  by  a  number  equal  to  the  sum  of  the  silica 
molecules  necessary  to  change  A  and  C  to  feldspars  and  F  to  the  metasilicate. 

According  to  this  system  (see  Fig.  43  and  rule  11)  a  rock  may  be  represented  by  a 
formula  similar  to  the  following : 

S59  Os-5  C12-5/4  /l8-3- 

This  indicates  a  rock  in  which  there  is  59  per  cent,  (molecular)  of  SiO2,  while 
(Na,  K)r  A12O4:  CaAl2O4:  (Fe,  Mg,  Ca)O  =  3.5: 12.5: 4,  that  is,  its  plagioclase  is  rather 
rich  in  lime  and  forms  (3.5  +  12.5  =  )16  parts  of  the  20  into  which  the  rock  was 
divided,  or  80  per  cent,  of  the  whole.  The  basic  constituents  constitute  the  other 
20  per  cent.  We  also  have  the  relation  Na2O:K2O  =  8.3:1.7,  that  is,  soda  is  much 
more  abundant  than  potash,  which  was  to  be  expected  in  a  plagioclase  rock.  The 
rock  is  a  gabbro-diorite. 

Numerous  graphic  methods  for  representing  the  chemical  compositions  of  rocks 
have  been  devised,  but  only  the  simplest  will  be  given  here.     In  a  diagram  either  the 


FezOs 


AhOa 


KjO  NazO  CaO  ^^~.  ~..~. 

FIG.  40. — Graphical  representation  of  the  composition  of  a  granite  from  Hauzenberg, 
near  Passau.     (After  Rosenbusch.) 

analysis  recalculated  to  100  per  cent.,  the  molecular  per  cent.,  or  the  metal  atom  per 
cent,  may  be  used.  For  example,  in  a  parallelogram  10  cm.  long  (  =  100  per  cent.) 
percentages  may  be  shown  in  millimeters,  the  difference  in  the  proportions  in  different 
rocks  giving  figures  which  enable  one,  with  a  little  practice,  to  obtain  a  very  good 
impression  of  the  composition  of  the  rock.  Thus  a  granite  from  Hauzenberg  near 
Passau  is  shown  in  this  manner  in  Fig.  40.  The  upper  rectangle  represents  the  weight 
percentage,  the  middle  the  molecular  percentage,  and  the  lower  the  metal-atom  per- 
centage corresponding  to  the  analysis  below.  This  is  recalculated  to  100  per  cent., 
water  and  the  minor  constituents  being  disregarded. 


II 


III 


SiO2 

73  83 

82  1 

70.4 

A12O3  

12.30 

8.1 

13.9 

Fe2O3.... 

4  18 

1  7 

2.9 

CaO 

0  94 

1  1 

1  0 

Na2O  

2.22 

2.3 

3.9 

K2O  

6  53 

4  7 

7.9 

Totals..  . 

100.00 

100.0 

100.0 

Another  diagram  for  representing  rock  analyses,  suggested  by  Michel-LeVy  and 
modified  by  Brogger,  consists  of  four  axes  radiating  from  a  center  (Figs.  41  and 


72 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


42).  The  SiO2  content  is  plotted  in  millimeters  upon  the  horizontal  line,  one-half  to 
the  right  and  one-half  to  the  left  of  the  center.  The  other  constituents  are  indicated 
by  proper  intercepts  on  the  other  axes,  the  points  so  determined  being  connected 


SiO* 


FIG.  41. — Graphical  representation  of  the  composition  of  the  granite  from  Hauzen- 
berg,  near  Passau.     (After  Michel-L6vy.) 

by  lines.     Fig.  41  represents  the  percentage  weights  in  the  Hauzenberg  granite, 
Fig.  42  those  in  a  gabbro  from  Radautal  i.  H. 

As  mentioned  in  rule  11  above,  the  rock-types  computed  by  Osann's  method  may 
be  indicated  by  points  in  a  triangle,  the  apices  representing  20a,  20c,  and  20f.     If 


CaO 


MgO 


FIG.  42. — Graphical  representation  of  the  composition  of  a  gabbro.     (After 

Michel-Levy.) 

the  values  of  a,  c,  and  /  are  laid  off  upon  the  medial  lines,  a  point  is  obtained  for  each 
rock  of  definite  chemical  composition.  The  example  given  on  page  71,  03.5,  012.5,  /-4, 
is  shown  by  the  dot  in  Fig.  43. 


FIG.  43. — Graphical  representation  of  a  gabbro-diorite.     (After  Osann.) 

The  system  of  Cross,  Iddings,  Pirsson,  and  Washington,  in  which  the  chemical 
types  and  textures  of  rocks  are  represented  by  stereotyped  "index  words,"  is  much 
too  complicated  and  voluminous  to  be  given  here,  even  in  outline. 


V.  ROCK  WEATHERING 

LITERATURE 

See  the  bibliography  given  by  JOH.  WALTHER:    "Lithogenesis  der  Gegenwart." 

Jena,  1894. 
M.  BAUER:  "Beitrage  zur  Geologic  der  Seychellen,  insbesondere  zur  Kenntnis  des 

Laterits."     Neues  Jahrb.,  1898,  II,  163. 
G.   BISCHOP:  "Lehrbuch  der  chemischen  und  physikalischen   Geologic."     4  vols., 

Bonn,  1847-1855. 

'A.  DAUBREE:  "Les  eaux  souterraines  de  1'epoque  actuelle."     Paris,  1887. 
A.  HEIM:  "Handbuch  der  Gletcherkunde."     Stuttgart,  1885. 
E.JW.  HOFFMANN:  "tlber  den  Einfluss  gewohnlichen  Wassers  auf  Silikate."    Leipzig, 

1882. 
T.  STERRY  HUNT:  "The  Decay  of  Rocks  in  Mineral  Physiology  and  Physiography." 

Boston,  1886. 
A.  JOHNSTONE:  "On  the  Action  of  Pure  Water  and  Water  Saturated  with  Carbonic 

Acid  Gas  on  the  Minerals  of  the  Mica  Family."     Quart.  Jour.  Geol.  Soc.,  London, 

XLV  (1889),  363. 

GEO.  P.  MERRILL:  "Rocks,  Rock-weathering,  and  Soils."     New  York,  1906. 
A.  PENCK:  "Morphologic  der  Erdoberflache."     Leipzig,  1894. 
F.  V.  RICHTHOFEN:  "Fiihrer  fur  Forschungsreisende."     Berlin,  1901. 
H.  ROSLER:  "Beitrage  zur  Kenntnis  einiger   Kaolinlagerstatten."     Neues  Jahrb., 

B.B.  XV  (1902),  231. 
J.  ROTH:  "Allgemeine  und  chemische  Geologic."     Bd.  I,  "Bildung  und  Umbildung 

der  Mineralien."     Berlin,  1879. 
I.  C.;  RUSSELL:  "Subaerial  Decay  of  Rocks  and  the  Origin  of  the  Red  Color  of  Certain 

Formations."     Bull.  52,  U.  S.  G.  S.,  Washington,  1889. 

P.'TREITZ:  "Was  ist  Verwitterung."     C.R.I,  confer,  intern.  agrogSol.,  1901,  131. 
J.  M.  VAN  BEMMELEN:  "Les  divers  modes  de  decomposition  des  roches  silicatees 

dans  la  croute  terrestre."     Arch,  neerl.  des  sciences  exactes,  XV  (1910)  (2),  284. 

Weathering  in  General. — The  term  weathering  is  used  to  de- 
scribe all  alterations  brought  about  by  the  atmosphere  and  the 
agents  present  in  it,  and  by  organisms  at  the  surface  and  within 
the  lithosphere.  The  action  of  weathering  is  in  part  chemical 
and  in  part  mechanical.  It  primarily  destroys  the  original  rocks, 
whose  constituents,  after  more  or  less  separation  by  wind  and 
water,  are  finally  deposited  to  form  the  sedimentary  rocks. 
Weathering  acts  as  a  leveler  of  the  relief  of  the  earth,  and  is  con- 
fined to  the  parts  adjacent  to  the  surface. 

The  term  replacement,  on  the  other  hand,  includes  alterations 
not  due  to  the  action  of  the  atmospheric  agents.  The  changes, 
therefore,  do  not  begin  at  the  surface,  and  are  not  confined  to  the 

73 


74  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

upper  strata.  Other  changes  are  produced  by  diagenesis  (Gr. 
5id,  after,  yevevis  origin),  or  the  re-formation  by  the  hydrosphere  of 
the  newly  deposited  products  of  weathering,  during  and  directly 
after  the  sedimentation. 

The  action  of  the  atmospheric  agents  is  chiefly  regional.  It 
is  dependent  upon  the  moisture  content  and  temperature  of  the 
air,  as  well  as  upon  the  content  of  chemically  active  agents.  The 
latter  are  usually  uniformly  distributed  but  may  be  more  abun- 
dant locally,  for  example  near  the  sea  or  near  recently  active  vol- 
canoes. Also,  certain  agents  derived  from  organisms  are  active, 
either  by  their  life-processes  or  by  their  decay.  There  are  to  be 
distingushed,  therefore,  physical,  chemical,  and  organic  weathering. 
In  most  cases  all  of  these  processes  act  together. 

Physical  Weathering. — The  phenomena  of  physical  weathering 
are  seen  best  in  arid  regions,  where  insolation  (Lat.  in,  in,  sol,  sun) 
and  great  temperature  differences  break  compact  rocks  into  great 
blocks  and  cause  the  exfoliation  of  granite  and  sandstone.  The 
great  sand  mass,  so  characteristic  of  the  desert,  is  due  primarily 
to  the  different  behavior  of  different  minerals  under  temperature 
changes. 

While  chemical  weathering  is  much  more  active  than  physical 
weathering  in  damp,  tropical  climates,  the  latter  distinctly  pre- 
dominates in  the  temperate  and  frigid  zones.  In  the  temperate 
zone  the  water  circulating  in  the  capillaries  of  the  rocks  is  an  impor- 
tant factor  in  their  destruction.  In  fact  the  amount  of  water 
absorbed  by  a  rock  is  taken  into  consideration  in  determining  its 
commercial  value.  For  example,  the  freezing  of  water  in  porous 
sandstones,  limestones  with  clayey  layers,  etc.,  may  make  them 
friable,  or  may  even  totally  destroy  them.  Physical  weathering 
is  especially  active  in  high  mountain  regions,  on  account  of  the 
occurrence  of  great  variations  in  temperature. 

Chemical  Weathering. — Chemical  weathering  is  usually  accom- 
panied by  physical  and  organic  weathering,  and  is  dependent 
upon  moisture  and  temperature.  Where  the  evaporation  of  the 
rainfall  is  greatly  retarded  by  a  thick  cover  of  vegetation,  as  in 
the  tropics,  it  is  of  great  importance. 

Water  falling  through  the  atmosphere  takes  up  certain  chemi- 
cally active  substances  even  from  the  purest  air  of  high  mountains, 
and  while  these  substances  are  generally  present  in  small  amounts, 


ROCK  WEATHERING 


75 


they  are  of  great  importance.  Besides  the  predominating  content 
of  oxygen,  CO2  as  well  as  traces  of  HC1  and  H2SO4  are  always  pres- 
ent, and  even  after  the  water  has  passed  through  fresh,  sulphur- 
free  silicate  rocks,  a  certain  amount  of  chlorides  and  sulphates 
still  remains. 

Geologists  have  been  inclined  to  over-estimate  greatly  the  importance  of  chemical 
weathering,  and  all  the  products  of  rock-decomposition  have  been  continually  con- 
founded with  weathering  products,  rendering  very  difficult  a  general  conception 
of  the  complete  process  of  rock  alteration.  Certain  rocks  undoubtedly  have  been  more 
or  less  completely  dissolved  by  weathering.  Thus,  rock  salt  occurs  on  the  surface 


FIG.  44. — Karst  topography.     Wiesalpe,  Dachstein.     (After  F.  Simony.) 

only  in  arid  regions,  and  the  so-called  "gypsum  chimneys"  are  simply  pits  dissolved 
in  fractured  gypsum-rock.  Limestone  also  is  more  or  less  easily  soluble,  and  the  water 
from  melting  snow  or  glacial  ice,  in  some  cases,  corrodes  the  underlying  rock  until  its 
surface  resembles  a  stormy  sea  turned  to  stone  (Fig.  44).  Further,  certain  peculiar 
funnel-shaped  depressions  of  the  Karst,  called  dolinas,  belong  here,  and  the  accumula- 
tions of  red  clay,  terra  rossa,  which  occur  within  them  are  doubtless  the  residues  from 
the  dissolved  limestone.  "Geologic  pipe  organs"  have  a  similar  origin.  These  are 
vertical  cylinders  which  occur  in  limestones,  especially  where  the  surface  above  was 
heavily  forested. 

Recent  investigations  make  it  appear  very  doubtful  whether  all  large  limestone 
caves  are  due  to  solution.  Such  caves  usually  occur  in  coral  limestones,  and  it  is 
worthy  of  note  that  entirely  analogous  openings  are  found  in  recent  coral  masses  where 
they  represent  original  gaps  in  the  reef.  Like  the  limestone  caves,  the  bottoms  of  these 
gaps  are  covered  with  terra  rossa  or  similar  cave-loam.  Coral  islands  in  general  show 
such  deposits,  consisting  of  drifted-in  laterite  and  weathered  volcanic  ashes  and 
pumiceous  sands.  The  presence  of  these  floor  deposits  appears  to  be  an  argument 
against  leaching  action,  for  moving  water  would  have  carried  away  the  fine  mud  of 


76  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

which  they  are  composed.  Stalactites  certainly  indicate  extensive  solution,  but  they 
tend  rather  to  close  up  large  caverns  than  to  produce  them.  In  open  spaces  the  dis- 
solving power  of  water  is  lessened  by  evaporation,  loss  of  carbonic  acid,  and  the  action 
of  organisms,  giving  rise  to  the  formation  of  sinter.  All  caverns  in  limestone,  there- 
fore, cannot  be  ascribed  to  atmospheric  agencies,  although  some  undoubtedly  represent 
widened  fissures. 

The  term  chemical  weathering,  in  the  strict  sense,  is  less 
commonly  applied  to  the  solution  of  entire  rock-complexes  than 
to  the  alteration  by  atmospheric  agents  of  the  primary  rock-form- 
ing minerals.  By  its  action,  sterile  rock  is  changed  to  fruitful, 
arable  land,  and  its  study  is  the  foundation  for  the  study  of  the 
soil. 

Chemical  weathering  is  due  primarily  to  the  action  of  vadose 
waters,  and  since  the  activity  of  the  latter  depends  especially 
upon  climatic  conditions,  chemical  weathering,  in  general,  depends 
upon  climate.  It  is,  therefore,  as  has  already  been  mentioned,  a 
regional  phenomenon.  Under  similar  climatic  conditions,  the 
same  kinds  of  rocks  everywhere  produce  approximately  the  same 
kinds  of  weathering  products.  It  is  hardly  necessary  to  point 
out  that  chemical  weathering  is  more  intense  in  moist,  warm 
climates  than  in  those  that  are  dry  and  cold,  but  the  nature  of  the 
action  and  the  new  minerals  produced  differ  but  slightly. 

Mineralogists  and  geologists  have  long  been  accustomed  to  ascribe  practically  all 
alterations  of  minerals  to  vadose  waters,  and  this  is  especially  true  of  the  alteration 
of  anhydrous  minerals  to  hydrous.  A  closer  examination,  however,  shows  that  numer- 
ous complicated  processes  which  have  been  ascribed  to  atmospheric  agents,  actually 
are  always  intimately  connected  with  volcanic  phenomena,  and  that  certain  altera- 
tions ascribed  to  weathering  are  simply  local  and  not  regional  occurrences. 

For  example,  one  of  the  most  important  of  these  non-regional  alteration  processes 
which  has  been  incorrectly  ascribed  to  weathering,  is  the  formation  of  kaolin  from 
feldspar  rocks.  A  close  examination  will  show  a  series  of  phenomena  proving  the 
incorrectness  of  former  conceptions  as  to  its  origin.  Kaolin  always  appears  in  very 
irregular,  isolated  masses  surrounded  by  normally  weathered  granite.  It  is  usually 
white  and  forms  a  marked  contrast  to  the  rust-like  products  of  weathering.  Further- 
more, it  is  nearly  or  entirely  free  from  potash,  and  always  entirely  free  from  apatite. 
The  salts  essential  to  plant  growth  being  thus  wanting,  kaolin  soil  is  extremely  unpro- 
ductive, differing  markedly  from  the  soil  from  weathered  granite,  which  is  very  fertile. 
It  is  true  that  certain  local  conditions  may  produce  a  much  more  intense  leaching  than 
usual,  whereby  the  residual  deposits  will  differ  from  the  ordinary  rusty  materials. 
Such  is  the  case  in  the  alterations  produced  in  rocks  by  overlying  peat  bogs,  whose 
organic  acids,  acting  as  reducing  agents,  carry  away  in  solution  all  of  the  iron  content. 
In  this  manner  white  weathering  products,  in  many  cases  very  kaolin-like  in  ap- 
pearance, are  formed,  and  these,  without  careful  examination,  might  be  identified  as 
kaolin.  The  fact  that  the  granite  in  the  more  important  deposits  is  completely  kao- 
linized  to  depths  far  beyond  that  reached  by  the  atmospheric  agents  under  any 
circumstance,  and  that  the  deposits  nearly  always  contain  new  minerals  (Ger.  Neu- 


ROCK  WEATHERING  77 

bildungeri)  such  as  tourmaline,  topaz,  fluorite,  scapolite,  pyrite,  and  siderite,  which 
certainly  do  not  normally  crystallize  from  vadose  water,  is  disregarded;  as  is  the  fact 
that  bog-weathering  never  produces  aggregates  having  the  composition  and  properties 
of  kaolin. 

Hereby  we  come  to  a  fundamental  difference  between  the  vari- 
ous products  of  rock  alteration.  Kaolin  and  analogous  products 
of  decomposition  are  crystalloids,  while  the  chief  weathering  prod- 
ucts of  feldspars  and  other  silicates  are  unquestionably  colloids 
or  gels.  So  far  as  is  determinable  with  certainty,  normal  weather- 
ing products,  in  the  main,  are  amorphous,  and  the  property  of 
the  soil  to  adsorb  the  alkalies  and  the  alkaline  earths  with  great 
ease,  and  just  as  readily  give  them  up  again  to  plants,  is  a  prop- 
erty characteristic  of  colloidal  substances  and  marks  an  especially 
important  difference  between  the  products  of  weathering  and 
kaolin.  Finally,  if  plants  could  remove  all  of  the  alkali  and 
lime  from  a  soil,  leaving  only  a  pure  aluminium  silicate,  it  could 
later  regain  these  salts  by  taking  them  from  solutions;  kaolin 
does  not  possess  such  a  power  of  adsorption. 

Because  the  alkalies  and  alkaline  earths  of  zeolites  may  be 
replaced  in  part,  the  conception  of  "  soil-zeolites "  was  introduced 
in 'soil-study,  but  although  true  zeolites  crystallize  very  readily, 
none  that  was  newly  formed  has  ever  been  observed  in  a  normal 
soil,  nor  has  the  probability  of  the  existence  of  such  crystalloids 
ever  been  shown  by  chemical  or  other  investigations.  Substances 
which  in  the  mineralogic  sense  may  properly  be  called  zeolites 
are  as  rare  among  normal  weathering  products  as  in  crystallized 
kaolin,  and  the  term  " soil-zeolite"  should  be  avoided,  for  it  pro- 
duces misconceptions  in  regard  to  chemical  weathering.  The 
constituents  of  the  soil  which  have  been  so  designated  are,  without 
doubt,  true  colloids. 

Great  confusion  likewise  has  been  produced  in  the  study  of 
weathering  by  the  double  meaning  given  to  the  word  day.  Under 
this  term  are  included  secondary  deposits  of  more  or  less  pure 
kaolin  as  well  as  the  finer  products  of  true  weathering.  The 
latter  material,  which  was  brought  together  by  running  water, 
consists  principally  of  the  colloidal  products  from  weathered 
silicates  and  of  fragments  of  quartz.  As  is  to  be  expected  from  the 
manner  of  their  origin,  kaolin  clays  are  purely  local  occurrences 
which  have  originated  from  the  destruction  of  primary  kaolin 
deposits.  The  clays  of  weathering,  on  the  other  hand,  are  normal, 
widely  distributed,  regional  sediments  which  usually  differ  greatly 


78          FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

from    true    kaolin    in    chemical    composition    and    mineralogical 
character. 

Finally,  as  the  last  straw,  the  term  laterite  (Lat.  later,  tile), 
which  is  properly  applied  to  the  normal  red  weathering  product 
of  the  tropics,  has  also  been  used  for  another  peculiar,  secondary 
substance  of  the  tropics.  The  latter  material  is  composed  chiefly 
of  colloidal  hydroxides  of  alumina  and  iron,  and  thus  differs  entirely 
from  normal  laterite,  whose  chemical  composition  is  not  greatly 
different  from  that  of  the  original  rock.  From  such  misconcep- 
tions originated  the  view  that  kaolin  represents  the  end-product 
of  weathering  in  temperate  and  cold  regions,  while  bauxite  is 
the  end-product  in  the  tropics,  where  the  warmer  atmospheric 
agents  had  much  greater  chemical  energy. 

The  principal  effect  of  chemical  weathering  is  the  destruction 
of  the  rocks.  There  are  formed  (1)  a  soluble  part — the  weathering 
solution — which  carries  off  the  basic  constituents  of  the  minerals 
by  the  aid  of  the  acids,  especially  carbonic  acid,  which  are  always 
present  in  vadose  water;  and  (2)  a  weathered  residue,  remaining 
where  the  weathering  took  place.  Since  nothing  is  lost  in  the  cycle 
of  katamorphism  and  anamorphism,  the  weathered  products  of 
the  primary  rocks  must  appear  again  somewhere  in  the  secondary. 
Thus  the  average  composition  of  argillites  and  sandstones  nearly 
corresponds  to  that  of  the  weathered  residuum  of  granite.  The 
solids  of  the  weathering  solutions  are  deposited  elsewhere,  the  cal- 
cium carbonate  contents  being  almost  entirely  separated  by  organ- 
isms, while  the  remaining  solids  appear  in  chemical  sediments 
such  as  those  in  the  rock-salt  formations. 

If  kaolin  is  due  to  weathering,  great  quantities  of  potash — 
twice  as  much  as  of  soda — should  have  been  leached  from  regions 
which  have  been  kaolinized,  for  about  90  to  95  per  cent,  of  the 
primary  rocks  are  of  the  composition  of  granite.  As  a  matter  of 
fact,  potash-salts  in  sediments  derived  from  weathering  solutions 
are  very  rare  as  compared  with  those  of  sodium.  If  the  bauxite 
of  the  tropics  were  produced  by  the  weathering  of  granite,  approxi- 
mately 7  per  cent,  of  potash  and  40  per  cent,  of  silica,  mainly 
from  feldspars,  must  have  been  carried  away.  Such  great  quanti- 
ties of  potash  are  not  found  in  the  precipitates  from  the  solutions, 
and  even  less  can  the  enormous  masses  of  dissolved  silica  be  found. 
It  is  true  that  deposits  of  silica,  separated  from  weathering  solu- 
tions by  the  action  of  organisms,  are  known  in  all  formations, 


ROCK  WEATHERING  79 

but  they  are  exceptional  occurrences  and  rarely  reach  any  consid- 
erable magnitude.  If  the  normal  product  of  tropical  weathering 
were  bauxite,  however,  two-fifths  of  all  sediments  should  consist 
of  silica  derived  from  the  solutions  of  weathering  by  organic  or 
chemical  precipitation.  That  this  is  contrary  to  the  actual  facts 
was  shown  in  the  chapter  on  the  formation  of  sediments. 

The  Weathering  Solutions. — The  material  leached  from  the 
rocks  by  vadose  waters  is  found  in  springs,  brooks,  streams,  seas, 
and  oceans,  and  from  the  composition  of  these  waters  the  action 
of  chemical  weathering  can  be  best  studied.  Waters  which  fall 
through  the  atmosphere  doubtless  always  carry,  besides  oxygen, 
certain  dissolved  acids,  which  are  the  primary  agents  of  chemical 
weathering.  In  all  running  waters,  besides  predominant  carbon- 
ates, there  are  present  sulphates  and  chlorides  whose  acid  radicals 
came  from  the  atmospheric  agents  and  whose  bases  came  from  the 
rocks  through  which  the  waters  passed.  Meteoric  water,  upon 
evaporation,  shows  hardly  a  trace  of  solid  matter,  but  this  is  always 
present  in  variable  amounts  in  springs,  brooks,  and  streams.  While 
the  destruction  of  the  rocks  by  these  weak  agents  is  very  slow, 
locally  the  processes  may  act  somewhat  more  intensely.  The  in- 
creased activity  of  the  solutions  when  strengthened  by  organic 
acids  has  already  been  mentioned.  The  weathering  is  most  active 
where  the  surface  rocks  are  rich  in  minerals  such  as  pyrite,  whose 
oxidation  sets  free  great  amounts  of  sulphuric  acid.  Such  weather- 
ing is  especially  characteristic  in  the  gossan  of  ore  deposits. 

The  composition  of  weathering  solutions  varies  greatly  in  different  regions. 
Where  the  climate  is  humid  the  percentage  of  dissolved  matter  is  less  than  in  dry  cli- 
mates on  account  of  the  greater  evaporation  in  the  latter.  The  salt-pans  of  the 
deserts  are  simply  concentrated  salt-solutions  whose  constituents  were  leached  from 
the  rocks,  and  whose  water  may  all  disappear  during  long  dry  periods.  In  areas  of 
granitic  rocks  in  humid  climates,  the  vadose  waters  are  especially  poor  in  solids,  the 
proportion  sinking  to  1  : 50,000;  ordinarily  the  percentage  in  springs  and  streams  is 
from  ten  to  twenty  times  as  great.  Carbonates  predominate  in  all  of  these  solutions, 
the  amount  averaging  70  to  80  per  cent,  of  the  total  residue.  The  remainder  consists 
of  about  10  to  15  per  cent,  of  sulphates,  5  to  10  per  cent,  of  chlorides,  and  a  very  small 
amount  of  silica,  perhaps  0.5  per  cent.  Of  the  total  dissolved  material,  70  to  80  per 
cent,  consists  of  calcium-salts  except  in  regions  which  are  entirely  granitic,  10  to  15 
per  cent,  of  magnesium-salts,  and  10  per  cent,  of  sodium-salts.  The  potash  contents 
is  rarely  greater  than  0.5  per  cent.  In  every  case,  the  weathering  solutions  are  very 
poor  in  silica  and  potash. 

The  composition  of  surface  waters  becomes  greatly  modified  in  arid  regions  when 

the  solid  constituents  are  concentrated  by  rapid  evaporation.     Accompanying  this 

concentration,  in  many  cases,  there  is  a  great  decrease  in  the  calcium  content,  the 

carbonate  being  precipitated  after  becoming  insoluble  in  the  concentrated  salts 

6 


80  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

solution.  Chlorides  and  sulphates,  in  rather  varying  proportions,  are  here  of  most 
importance,  the  Chelif  River  in  Algeria,  for  example,  containing  as  much  as  7  gm. 
residue  to  the  liter.  Of  this  residue,  carbonates  form  about  2  per  cent.,  sulphates  over 
50  per  cent.,  and  chlorides  40  per  cent.,  corresponding  to  25  per  cent,  each  of  calcium- 
and  magnesium-salts,  and  the  remainder  of  sodium-salts.  Here,  likewise,  silica  and 
potash  are  of  no  importance. 

Evaporation  increases  the  salt  content  of  the  true  salt-seas  of  the  steppes  until  the 
waters  are  saturated.  Soda  and  magnesia  are  of  prime  importance,  although  they 
occur  in  decidedly  variable  proportions,  while  lime  is  relatively  unimportant,  having 
been  almost  entirely  precipitated  as  gypsum.  Bitter-seas,  rich  in  magnesia,  may  con- 
tain as  much  as  30  per  cent,  of  dissolved  constituents.  Other  materials  of  weath- 
ering, such  as  the  salts  of  strontium,  which  ordinarily  are  hardly  perceptible,  may 
occur  in  distinctly  recognizable  amounts. 

It  is  worthy  of  note  that  phosphate  plays  no  role  in  vadose  waters.  "Practically 
all  of  the  apatite  of  the  original  rocks  remains  in  the  weathered  residue,  from  which 
it  may  be  withdrawn  later  by  vegetation.  On  the  other  hand,  nitrates  are  here  and 
there  present.  They  form  extensive  deposits  in  the  Chilian  pampas,  and  represent 
a  peculiar  development  of  salt-pans.  Their  origin  has  been  ascribed  to  frequent  and 
tremendous  electrical  discharges  which  oxidized  the  nitrogen  of  the  air. 

Borax-seas  and  borax  deposits  likewise  occur  in  deserts,  but  they  have  a  somewhat 
different  origin.  They  appear  to  occur  exclusively  in  volcanic  regions  where  boron- 
bearing  fumaroles  have  acted  upon  salt  deposits.  The  not  uncommon  cementation 
of  desert  sediments  with  silica,  however,  is  to  be  traced  to  vadose  waters,  whose  small 
silica  content  was  precipitated  in  the  general  concentration  of  the  weathering  solu- 
tions. By  far  the  most  silicified  rocks,  however,  were  altered  by  silica-rich  juvenile 
waters,  and  not  by  solutions  of  weathering. 

Running  waters  carry  their  dissolved  constituents  to  the  ocean,  and  thus,  by  the 
continual  addition  of  solutions,  even  though  very  dilute,  and  the  evaporation  of , 
pure  water,  they  have  gradually  become  perceptibly  salt.  Although  the  entering 
waters  come  almost  exclusively  from  humid  regions,  the  composition  of  the  sea  water 
differs  entirely  from  the  weathering  solutions  brought  in.  Most  noteworthy  is  the 
almost  complete  loss  of  carbonates.  These  were  partially  removed  by  precipitation 
as  oolite  when  the  solution  was  sufficiently  concentrated,  but  in  the  main  they  were 
deposited  as  organogenic  sediments,  having  been  taken  up  by  organisms  which  required 
calcium  for  their  hard  parts.  The  normal  residue  of  ocean  water  consists  of  not  quite 
80  per  cent,  sodium-salts,  about  15  per  cent,  magnesium-salts,  5  per  cent,  calcium- 
salts,  and  1  per  cent,  potash-salts.  These  are  combined  in  the  form  of  chlorides 
about  90  per  cent.,  and  sulphates  10  per  cent.  The  chief  constituent  is  sodium 
chloride,  although  in  ordinary  weathering  solutions  it  is  of  slight  importance. 

Exhaustive  investigations  of  weathering  solutions  show  with  absolute  certainty 
that  normal  weathering  has  never  altered  granite  to  kaolin,  nor  even  to  bauxite. 
This  is  in  harmony  with  the  conclusion  already  drawn  from  geologic  relationships. 

The  Weathered  Residues. — A  study  of  the  weathered  residues 
leads  to  exactly  the  same  result  as  did  that  of  weathering  solutions. 
This  is  shown  by  a  comparison  of  the  composition  of  a  fresh  granite, 
for  example  that  from  Altmittweida  in  Saxony  (I),  with  that  of  the 
granite-grush1  derived  from  it  (II). 

1  The  translator  has  taken  the  liberty  of  using  the  verb  grush,  meaning  to  crumble 
down,  to  disintegrate,  as  a  noun  for  the  finely  crumbled  but  not  dust-like  rock  called 
grus  by  the  Germans. 


ROCK  WEATHERING 


81 


SiOt 

AhOs    1 

Ft 

MgO 

CaO 

NasO 

KiO 

HiO 

I 

73.43 

14.38 

2 

19 

0.22 

j 

0.68  : 

3.03 

6.07 

0  96 

II 

73  84 

14.62 

1 

?,8 

0.35 

0.37 

1.56 

5  98 

2  85 

These  analyses  show  that  only  lime  and  soda  have  been  with- 
drawn in  any  considerable  amount  from  the  rock  by  the  atmos- 
pheric agents.  The  increase  in  magnesia  in  the  weathered  product 
is  probably  an  error  in  the  determination.  The  noteworthy 
feature,  however,  is  the  fact  that  the  proportions  of  silica,  alumina, 
and  water  in  the  second  analysis  are  entirely  different  from  those 
in  kaolin.  Kaolinized  granite  contains  at  least  one  and  one-half 
to  two  times  as  much  alumina  and  about  five  times  as  much  water 
as  the  granite-grush  above. 

Granite-grush  may  be  sorted  later  by  wind  and  flowing  water, 
and  then  be  deposited  as  sandstone  or  calcareous  argillite; 
the  former  consisting  essentially  of  quartz,  the  latter  of  the  col- 
loidal weathering  products  of  the  rock-forming  silicates.  The 
silica  content  is  large  in  sandstones,  and  few  conclusions  can  be 
drawn  from  these  rocks  in  regard  to  the  course  of  the  weathering 
of  the  silicates.  A  study  of  the  finer  material  which  was  washed 
out  and  deposited  as  argillite  gives  better  results.  For  example, 
a  very  plastic  kaolin  from  Klingenberg  on  the  Main  (I)  and  a  nor- 
mal sedimentary  clay  from  the  English  coal  formation  at  Frank- 
land  (II)  have  the  following  analyses : 


SiO2 

1 

AhOs 

FezOs 

MgO 

CaO 

NazO 

K20 

HjO 

I                              49  37 

30  10 

3  9 

0  38 

16  24 

II  61  91 

20  73 

5  01 

0  59 

0  5 

0  25 

3  16 

6  73 

An  inspection  of  these  and  the  analyses  previously  given,  shows  without  question 
that  kaolin  and  bauxite  are  not  normal  weathering  products.  The  same  conclusion 
is  reached  from  an  examination  of  the  chemical  composition  of  the  soils  produced 
by  organic  weathering.  The  primary  result  of  all  weathering  processes  is  the  removal 
of  the  lime,  magnesia,  and  soda  by  vadose  waters.  The  greater  part  of  the  potash 
remains  in  the  soil,  to  be  very  gradually  withdrawn,  though  seldom  entirely,  by  plants. 
The  colloidal  alteration  products  of  the  silicates  which  were  formed  by  chemical 
weathering  are  to  be  recognized  from  the  physical  properties  of  the  soil  rather  than  by 
direct  observation.  These  colloids  form  but  very  thin  films  upon  the  surfaces  of  the 
different  minerals,  so  that  a  completely  disaggregated  granite-grush  still  permits  the 
main  features  of  its  chemical  composition  to  be  clearly  recognized.  The  potash  of 
the  feldspar  is  very  slowly  leached  from  the  thinnest  beds,  and  is  made  accessible  to 
plants  through  colloidal  aluminium  silicates.  The  leached  soil  may  be  regenerated 


82  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

artificially  through  the  addition  of  potassium-salts,  which  become  absorbed  by  the 
colloids,  or  naturally  by  allowing  the  land  to  lie  fallow.  In  the  latter  case  the  potash 
of  the  smallest  feldspar  particles  in  the  soil  is  made  available  by  the  superficial  altera- 
tion of  this  mineral  to  colloids,  which  later  give  up  their  content  of  alkalies  and  alkaline 
earths  to  plants.  The  slow  process  of  true  weathering  is  thus  entirely  different  from 
the  relatively  rapid  and  radical  process  of  kaolinization,  and  this  difference  is  shown 
not  only  by  the  greater  or  lesser  rapidity  of  the  processes  themselves,  but  by  the 
nature  of  their  end-products,  kaolin  and  colloids. 

Climatic  Zones  of  Weathering. — The  difference  in  the  character 
of  weathering  in  humid  and  dry  climates  was  mentioned  under  the 
heading  of  weathering  solutions.  Here  the  influence  of  hot,  tem- 
perate, or  cold  climates  upon  the  final  character  of  the  weathered 
products  will  be  described.  Qualitatively  the  weathering  solu- 
tions are  the  same  in  all  zones,  and  the  differences  in  the  character 
of  the  products  are  very  small.  The  external  rather  than  the  in- 
ternal characteristics  of  the  weathered  residues  are  of  importance. 

The  intensity  of  chemical  weathering  depends  upon  the  hu- 
midity and  temperature  of  the  atmosphere.  It  acts  rapidly  and 
deeply  in  the  humid  climate  of  the  tropics,  but  only  superficially 
in  the  arctics.  The  differences  in  coloring  in  different  climatic 
zones  are  especially  noteworthy.  In  the  polar  regions  the  weath- 
ered products  are  light-colored  to  white,  in  the  temperate  zone 
they  are  typically  rust-colored,  while  in  the  tropics  they  usually 
have  the  red-brown  to  red  color  of  laterite.  This  difference  is  due 
primarily  to  the  extreme  slowness  of  weathering  processes  in  cold 
climates,  the  oxygen  of  the  vadose  waters  being  extremely  in- 
active here,  and  the  iron  oxide  is  removed  from  the  rocks  by  solu- 
tions as  fast  as  it  is  formed.  In  the  temperate  zone,  the  iron  is 
precipitated  as  colloidal  iron  hydroxide  or  rust;  the  higher  the 
temperature  of  the  surroundings,  the  less  water  in  the  iron  hydrates 
and  the  redder  the  color.  That  the  intensity  of  the  color  of  any 
weathered  rock  depends  upon  the  percentage  of  iron  in  it,  is  self- 
evident,  for  example  a  gabbro  or  trap  normally  weathers  darker 
than  a  granite,  but  the  degree  of  color  of  any  particular  rock- 
type  indicates  its  climatic  zone. 

It  may  here  again  be  mentioned  that  the  name  laterite  is  used 
in  the  tropics  not  only  for  normal  weathering  products,  but  also 
for  peculiar  deposits  which  consist  primarily  of  colloidal  aluminium 
and  iron  hydrates.  While  always  local  in  occurrence,  some  of 
these  deposits  are  of  considerable  size,  and  many  of  them  grade 
into  commercially  valuable  masses  of  bauxite  and  limonite.  These, 


ROCK  WEATHERING  83 

however,  as  has  been  pointed  out  several  times,  are  entirely  anom- 
alous formations,  and  while  in  some  cases  they  can  be  distinctly 
recognized  as  alteration  products  of  different  igneous  rocks,  they 
certainly  cannot  be  considered  to  be  their  normal  weathering  prod- 
ucts. In  many  places  analogous  masses  are  found  which  un- 
doubtedly are  new  formations  (Neubildungeri),  being  in  part 
superficial  incrustations,  in  part  aggregates  or  fragments  embedded 
in  limestone.  In  their  irregular  form  and  their  pisolitic  texture, 
which  in  many  cases  is  distinct,  they  resemble  the  bauxite  deposits 
of  the  sediments  of  former  geologic  periods.  In  general,  also, 
they  differ  from  normal  weathering  products  in  the  irregularity 
of  their  bedding. 

Laterite,  the  normal  weathered  material  of  the  tropics,  covers  a  considerable  part 
of  the  earth's  surface,  and  upon  much  of  it  there  is  a  dense  growth  of  vegetation. 
The  anomalous  deposits,  however,  have  a  different  appearance,  for  heavy  vegetation 
cannot  grow  from  soil  composed  t)f  pure  aluminium  hydrate  and  quartz,  since  the  salts 
necessary  for  plant  nourishment  are  wanting. 

The  red  color  of  the  weathered  material  in  the  tropics  is  undoubtedly  the  result 
of  a  warm  climate,  and  it  is  as  justifiable  to  conclude  from  the  red  color  of  a  sediment 
that  a  warm  climate  existed  during  its  deposition,  as  it  is  from  the  presence  of  tropical 
flora  and  fauna.  Similarly,  very  light-colored  sediments  generally  indicate  a  former 
cold  climate.  The  color  of  the  weathered  material  in  the  different  climatic  zones, 
therefore,  serves  as  an  important  aid  in  the  study  of  the  earth's  history. 

Chemical  Weathering  of  Former  Periods. — The  course  of 
chemical  weathering,  as  it  occurs  at  the  surface  of  the  earth  at 
the  present  time,  may  be  accurately  followed.  The  question 
arises,  may  not  the  climatic  conditions  in  former  geologic  periods 
have  been  so  greatly  different  that  ordinary  atmospheric  weather- 
ing yielded  products  which,  at  the  present  time,  form  only  under 
very  exceptional  circumstances?  The  carbonic  acid  which  is  now 
stored  in  the  limestones  and  dolomites,  the  carbon  of  organisms, 
and  the  chlorine  which  is  present  in  the  waters  of  the  ocean,  were 
all  doubtless  taken  from  the  primordial  atmosphere.  Our  present 
atmosphere  contains  about  one-thirtieth  of  1  per  cent,  of  carbon 
dioxide,  but  even  a  very  slight  increase  in  the  amount  would 
make  it  impossible  for  the  higher  animals  at  least,  to  live.  If  all 
of  the  carbon  dioxide  of  the  limestones  and  dolomites  and  the  chlo- 
rine of  the  chlorides  had  been  contained  in  the  original  air,  there 
would  have  been  present,  in  far  remote  geologic  periods,  such  enor- 
mous amounts  of  these  agents,  that  it  would  have  been  neces- 
sary for  the  respiratory  organs  of  animals  to  have  been  very  differ- 


84  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

ently  organized.  Such  an  active  atmosphere,  also,  probably 
would  have  caused  intense  alteration  of  the  rocks,  such  as 
occurs  only  locally  at  present. 

The  sequence  of  weathering  can  be  followed  in  the  oldest 
geologic  periods  with  nearly  the  same  accuracy  as  it  can  at  the  pres- 
ent time.  The  mechanical  sediments  show  the  composition  of  the 
residuals  of  weathering,  while  the  chemical  precipitates  show  at 
least  the  fundamentals  of  that  of  the  weathering  solutions.  Both 
mechanical  and  chemical  sediments,  from  the  oldest  Cambrian 
formations  to  the  present,  show  the  same  characters  throughout, 
consequently  there  is  no  reason  to  suppose  that  any  notable  change 
has  taken  place  in  the  character  of  the  atmosphere  during  that 
time. 

Thus  there  is  an  apparently  unbridgeable  difference  between 
direct  observation  and  speculation.  But  this  difficulty  vanishes 
when  a  factor,  until  now  neglected,  is  taken  into  account,  namely 
the  influence  of  vulcanism  in  preserving  the  equilibrium  in  the 
composition  of  the  atmosphere.  The  carbon  dioxide,  brought 
from  the  interior  of  the  earth,  continues  to  replace  that  which  is 
being  stored  in  the  carbonates  and  other  organic  sediments. 

During  periods  of  especially  great  volcanic  activity  there  resulted  an  atmosphere 
high  in  carbon  dioxide.  This  led  to  the  development  of  unusually  abundant  vegeta- 
tion and  the  reestablishment  of  the  equilibrium.  The  most  extensive  organic  sedi- 
ments, therefore,  are  found  in  connection  with  formations  which  are  characterized 
by  exceptionally  intense  volcanic  activity;  for  example,  the  Carboniferous  and  the 
Tertiary.  From  such  observations  the  conclusion  is  justifiable  that  while  the  amount 
of  carbon  dioxide  in  the  atmosphere  varied  during  past  geologic  periods,  the  propor- 
tion was  never  very  different  from  that  existing  at  the  present  time.  So  far  as  can  be 
determined  from  the  formations  remaining,  there  never  was  enough  carbon  dioxide  in 
the  air  at  one  time  to  produce  the  limestone  formations  now  found. 

In  the  same  manner,  the  chlorine  content  of  the  atmosphere  remained  constant. 
Free  hydrochloric  acid  is  added  by  volcanic  eruptions,  the  volcano  Purace  in  Colombia    : 
alone  giving  off  over  30,000  kg.  daily. 

Organic  Weathering. — The  processes  of  organic  weathering, 
which  generally  accompany  and  are  usually  intimately  connected 
with  the  chemical  processes,  are  somewhat  different.  Vegetation 
removes  from  the  rocks,  especially  from  the  comminuted  products 
of  chemical  weathering,  substances  which  are  not  soluble  in  the 
atmospheric  agents.  Chemical  and  physical  weathering  make 
the  constituents  of  the  rocks  more  available  to  plants,  and  these, 
by  the  secretions  of  their  roots  and  the  cooperation  of  soil  bacteria, 
extract  the  substances  necessary  to  their  existence — primarily 


ROCK  WEATHERING  85 

potash,  lime,  and  phosphoric  acid.  Although  lichens,  moss,  and 
the  like  are  able  to  obtain  their  nourishment  directly  from  rocks, 
any  considerable  number  of  the  higher  plants  can  obtain  the  neces- 
sary salts  only  from  the  colloidal  weathering  products. 

Different  plants  withdraw  the  salts  from  the  soil  in  different  proportions,  and 
thus,  while  they  remove  certain  constituents,  they  produce,  by  the  destruction  of  new 
material,  an  enrichment  in  substances  less  necessary  to  themselves.  This  fact  is  of 
great  importance  to  agriculture.  Where  chemical  weathering  is  slight,  the  nourishing 
salts  of  a  soil  are  frequently  exhausted.  But  this  exhaustion  is  only  apparent.  When 
the  fields  lie  fallow  for  a  time,  the  necessary  constituents  are  renewed  during  the 
period  of  predominating  chemical  weathering,  and  new  parts  of  the  soil  are  prepared 
for  the  use  of  the  vegetation. 

Rock-sculpture  by  Weathering. — Different  kinds  of  rocks  are 
affected  by  weathering  in  very  different  ways,  and  upon  this  de- 
pends, primarily,  the  relief  and  richness  of  form  of  mountains. 
Rocks  which  are  rich  in  glass  weather  much  more  easily  than  those 
that  are  holocrystalline,  and  porous  and  schistose  rocks  and 
those  that  are  internally  crushed  are  much  more  easily  destroyed 
than  those  that  are  compact.  The  alkali-rich  silicates,  especially 
nephelite  and  the  minerals  of  the  sodalite  group,  weather  espe- 
cially easily,  and  in  many  cases  are  dissolved  out  entirely  at  the  sur- 
face of  the  rocks.  The  weathered  surfaces  of  nephelite-syenites, 
therefore,  are  usually  pitted  and  corroded. 

In  other  cases,  rocks  which  in  the  fresh  state  appeared  homo- 
geneous throughout,  show  different  susceptibilities  in  different 
parts,  and  irregularities,  which  originally  were  not  visible,  may  be 
brought  out  distinctly  by  the  action  of  weathering.  The  weath- 
ered stratum  or  alluvium  (Lat.  eluere,  to  leach),  therefore,  does  not 
form  a  cover  of  uniform  thickness  parallel  to  the  surface, -but 
an  uneven  layer;  in  fissures  and  over  easily  decomposed  rocks 
the  action  has  taken  deep  hold,  over  resistant  rocks  there  is  only 
a  thin  cover. 

The  rock-sculptures  produced  by  the  strong  air  currents  of  the 
deserts  differ  entirely  from  those  produced  in  a  warm,  humid, 
tropical  region  with  dense  vegetation.  In  deserts  all  decomposi- 
tion products  are  blown  away,  and  the  fresh  rocks  are  exposed. 
In  the  tropics  most  of  the  rocks  are  porous  and  much  altered  to 
considerable  depths,  and  in  place  of  sharp  cliffs,  the  forms  are 
rounded,  and  fresh  rocks  seldom  appear  at  the  surface.  There 
is  likewise  a  characteristic  difference  between  the  weathered  forms 


86  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

on  medium  and  high  mountains,  the  material  on  the  latter  being 
carried  away  as  soon  as  it  is  formed. 

The    characteristic   partings   and   jointings   of   certain   rocks 


FIG.  45. — Platy  weathering  in  granite.     Rudolfstein,  Fichtelgebirge. 

(see  Sec.  X)  have  considerable  influence  on  the  weathered  forms. 
In  many  cases  these  joints  are  distinct  in  the  fresh  rocks  and  con- 
tinue downward  unchanged,  for  example,  in  basaltic  columns; 


FIG.  46. — Rock  stream.     Reichenbach,  Odenwald.     (Prof,  Dr.  Klemm,  photo.) 

in  others  they  appear  only  in  the  weathered  rocks  and  then  are 
gradually  lost  with  increasing  depth. 

Jointing  may  be  seen  in  many  granites.     At  the  surface  the  rocks 
have  a  more  or  less  thick-platy  parting  (Fig.  45),  but  as  the  dis- 


ROCK  WEATHERING 


87 


tance  from  the  weathered  surface  increases,  the  joints  are  farther 
and  farther  apart  (cf.  Fig.  6),  until  finally  homogeneous  rock 
is  reached.  But  even  in  this  compact  rock  there  are  invisible 
parting-planes,  called  rift  and  grain,  which  are  generally  due  to  a 
parallel  orientation  of  the  minerals,  and  which  permit  easy 
cleavage  in  those  directions.  The  rock  is  made  porous  along  these 
planes  by  weathering,  and  the  circulating  water  which  enters 
brings  out  the  superposed  plates.  A  rock  usually  possesses  several 
such  jointing  directions,  so  that  atmospheric  agents  may  round 
off  angular  fracture-surfaces,  and  produce  spheroidal  forms.  If 
the  weathered  material  between  the  harder  parts  is  carried  away, 
the  rock  may  become  so  incoherent  that  apparently  solid  rocks 
are  suddenly  precipitated  into  so-called  "  rock-seas "  or  "  rock- 
streams"  (Fig.  46). 


FIG.  47. — Spheroidal  weathering  of  diabase.     Fichtelberg,   Fichtelgebirge. 

Dr.  Klemm,  photo.) 


(Prof. 


The  weathering  agencies  tend  to  round  the  edges  of  rocks  of  uniform  texture  where 
they  are  cut  by  joints,  and  produce  forms  resembling  loaves  of  bread.  This  is  seen 
in  the  fractured  diabase  in  Fig.  47,  and  in  the  jointed  basaltic  columns  in  Fig.  48. 
On  the  other  hand,  rocks  which  are  internally  mashed,  as  are  the  central  Alpine 
granites,  fall  into  sand  and  grush  upon  weathering.  The  thinner  the  laminae  of  schis- 
tose rocks,  the  more  easily  do  they  weather,  consequently  true  schistosity  assists 
weathering  as  much  as  does  transverse  schistosity. 

Many  phonolites  show  a  thin-platy  parting  at  the  surface  (Fig.  49),  while  farther 
down  they  are  compact  but  have  a  rift  in  parallel  planes.  The  platy  partings  of  cer- 
tain quartz  porphyries,  on  the  other  hand,  were  not  caused  by  weathering  but  by  con- 
traction during  cooling,  which  in  many  cases  was  so  great  that  quartz  phenocrysts  were 
sheared  across  in  the  same  manner  as  are  olivine  crystals  hi  basalt,  parts  of  the 
same  crystal  occurring  in  adjacent  columns. 


88 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


The  first  indication  of  weathering  in  a  granite  is  generally  the  appearance  of  rusty 
infiltration  products.  These  may  be  present,  not  only  in  the  granite-grush,  but  even 
in  the  apparently  fresh  rocks.  They  penetrate  from  the  surface  downward,  as  if 


FIG.  48. — Spheroidal  weathering  of  basaltic  columns. 

(J.  Roth.) 


Schlossberg,  Aussig,  Bohemia. 


through  fissures,  and  form  a  yellowish  stain,  indicating  that  the  rock  is  porous,  or 
appear  in  the  form  of  dendrites  (Gr.  bkvbpov,  tree)  in  the  joints.  If  the  weathering 
penetrates  deeper  than  usual,  as  in  the  granite  of  the  Bavarian  Forest,  there  may  be 
found,  beneath  the  upper  rusty  layers,  completely  altered  sandy  beds,  still  showing  the 


FIG.  49. — Platy  weathering  of  phonolite.     Black  Hills,  S.  D.     (J.  D.  Irving.) 

original  granitic  texture  and  with  still  unoxidized  iron,  the  waters  having  lost  their 
oxygen  before  reaching  this  depth. 

Denudation. — The  forms  produced  by  weathering  are  made 
apparent  by  the  forces  of  denudation  (Lat.  denudare,  make  bare). 


ROCK  WEATHERING 


89 


These,  however,  do  not  simply  transport  the  weathered  material, 
but  they  attack  and  wear  away  the  country-rock  by  means  of  the 
fragments  carried.  The  thinner  the  cover  of  vegetation,  the 
stronger  the  denudation;  therefore  it  is  particularly  great  in  deserts, 
on  high  mountains,  and  in  the  polar  regions.  The  weathered 
forms  of  a  denuded  region  differ  according  to  the  agents.  Wind 
and  continental  ice  denude  the  entire  region  over  which  they  act, 
and  are  not  dependent  upon  the  slope  of  the  surface  as  are  flowing 
water  and  glacier  ice.  The  denuding  action  of  wind  is  called  defla- 
tion (Lat.  deflare,  blow  off),  that  of  flowing  water  erosion  (Lat. 
erodere,  gnaw  out),  that  of  the  sea  abrasion  (Lat.  abradere,  scratch 
off),  and  that  of  glaciers  exardtion  (Lat.  exarare,  plow  out). 


FIG.  50. — Corraded  granite.     Russian  Turkestan.     (Prof.  Dr.  Merzbacher,  photo.) 


Sand  masses  moved  by  desert  storms  may  wear  away  even  the  hardest  rocks. 
This  corrosion  (Lat.  corrado,  scrape  together)  may  gnaw  deep  holes  in  uniformly  granu- 
lar rocks  like  granite  (Fig.  50),  or  polish  compact  rocks,  or  pit  those  consisting  of 
granular  aggregates  of  minerals  of  different  hardnesses.  Furthermore,  flat,  arched 
facets  may  be  ground  on  the  larger  fragments  (Dreikanter} ,  or  the  surface  may  be 
gouged  and  smoothed  until  it  suggests  that  of  a  meteorite.  This  resemblance  is  made 
still  greater  by  a  glistening  black  crust  of  manganese  and  iron  hydroxides,  the  so-called 
desert  varnish.  The  wind  also  concentrates  the  larger,  harder  constituents  of  broken 
rocks  while  it  blows  away  the  more  easily  destroyed  interstitial  parts.  This  is  seen, 
for  example,  in  the  accumulations  of  flint  nodules,  fossils,  etc.;  the  so-called  desert 
pavements. 

The  residual  forms  produced  by  the  wind  depend  upon  the  character  of  the  rock. 
Compact  rocks  form  steep,  high  cliffs,  sharp  needles,  and  jagged  peaks,  while  horizon- 
tally bedded  rocks  become  flat  table-lands  with  surfaces  of  harder  strata.  If  these 
are  cut  by  sharp  fissures  and  cross-fractures,  typical  bad-land  topography  with 
characteristic  mushroom  forms,  results  (Fig.  51). 


90          FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


FIG.  51. — Bad-land   topography.     Washakie   Basin,    Wyoming.     (After   C.    King.) 


FIG.  52. — Bosses,  Grimsel  hospice.     Bernese  Oberland.     (Photo.  Photoglob.) 


ROCK  WEATHERING  91 

Denudation  by  continental  ice,  like  that  by  wind,  is  regional,  but  the  forms 
produced  are  entirely  different.     The  whole  weathered  surface  is  removed  by  the 


FIG.  53. — Grand  Canyon  of  the  Colorado. 

former,  and  the  fresh  rock  takes  on  predominatingly  rounded  forms,  the  so-called 
roches  moutonnees  (Fig.  52).     While  fresh  rock  may  be  exposed  by  the  sand-blasts  of 


FIG.  54. — Trass  deposits.     Tonnisstein,   Brohltal,  Eifel.     (After  Volzing.) 

the  desert  as  well  as  by  the  plowing  action  of  continental  ice,  the  surface  exposures 
are  different.  In  one  case  they  are  covered  by  the  peculiar  desert  varnish,  in  the  other 
polished  and  striated  by  the  scouring  of  the  ground-moraine.  In  neither  case  are  the 


92  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

new  deposits  confined  to  the  valley  bottoms;  they  also  occur  as  sand-glaciers  or 
moraines  high  up  in  the  mountains. 

Glacial  action  is  more  local  and  is  generally  confined  to  valleys.     It  produces  ero- 
sion forms  analogous  to  those  formed  by  continental  ice.     In  each  case  there  is  a 


FIG.  55. — Dolomite  ridges.     Bozen. 

tendency  to  round  corners,  smooth  out  the  sharp  and  hard  lines  of  the  landscape,  and 
widen  the' valleys.  $\ 

Denudation  by  means  of  flowing  water  is  entirely  different.     Here  everything 
moves  downward,  and  deposition  is  confined  to  the  valley  bottoms.     Where  there 


FIG.  56. — Silicified  baryte  dike.     Borstein,  Reichenbach,  Odenwald. 

is  a  great  difference  in  elevation  between  the  upper  stream  course  and  its  mouth,  the 
neighboring  rock  may  be  cut  into  deep  valleys  and  gorges.  An  extreme  case  is  the 
grand  canyon  of  the  Colorado  (Fig.  53)  which  has  a  depth  of  more  than  a  thousand 
meters.  Especially  steep  are  the  walls  of  water-cut  channels  when  the  materials  of 


ROCK  WEATHERING 


93 


which  they  are  composed  consist  of  porous  deposits  of  clay  or  tuff,  such  as  the  porous 
tuff  deposits  of  the  Eifel  (Fig.  54),  or  the  loess. 

Streams  with  a  lower  gradient  have  broader  valleys  with  great  masses  of  talus  at 
either  side,  and  the  larger  blocks  of  this  fallen  material  usually  preserve  their  angular 
outlines,  in  contrast  with  the  rounded  bowlders  formed  by  ice  or  flowing  water. 

Rocks  which  are  hard  and  able  to  withstand  chemical  weathering  are  least  cor- 


FIG.  57. — The  devil's  wall,  a  denuded  basalt  dike.     Oschitz,  Bohemia. 

roded,  so  that,  after  denudation,  they  stand  out  in  great  jagged  peaks,  like  the  dolo- 
mites of  South  Tyrol  (Fig.  55).  Even  at  lower  altitudes,  pegmatites  and  massive 
quartz  dikes  stand  out  prominently  above  the  surrounding  weathered  material 
(Fig.  56),  and  serpentine  forms  projecting  knobs.  Locally,  also,  steep-walled  dikes  of 
basalt  may  stand  out  from  more  friable  country-rock,  an  especially  grotesque  dike 
being  shown  in  Fig.  57.  Lamprophyres  generally  weather  more  easily,  and  therefore 
ordinarily  appear  as  shallow  trenches  instead  of  as  ridges. 


FIG.  58. — Map  of  the  south  coast  of  the  Isle  of  Arran.     The  trap  dikes  are  promi- 
nently exposed  by  abrasion. 

The  effects  of  abrasion  are  similar  to  those  of  erosion  but  naturally  are  confined  to 
seacoasts.  The  breakers  act  upon  the  softer  rocks  first.  If  these  are  horizontal, 
they  are  undermined;  if  they  run  at  an  angle  to  the  coast,  they  are  cut  into  deep  bays. 
An  excellent  example  of  the  latter  is  shown  on  the  south  coast  of  the  Isle  of  Arran  (Fig. 
58),  where  hundreds  of  trap  dikes  stand  out  like  walls  above  the  easily  weathered 
sandstone,  and  extend  far  into  the  sea. 


VI.  THE  NATURE  OF  THE  SEDIMENTS 

LITERATURE 

E.  AND  REE:  "Die  Diagenese  der  Sedimente,  ihre  Beziehungen  zur  Sedimentbildung 

und  Sedimentpetrographie."     GeoL  Rundschau,  II  (1911),  61,  117. 
Idem:  "Uber  Sedimentbildung  am  Meeresboden.  I.  Teil."     GeoL  Rundschau,  III 

(1912),  324. 
H.  BOEKE:  "Ubersicht  der  Mineralogie,  Petrographie  und  Geologie  der  Kalisalzlager- 

statten."     Berlin,  1910. 

G.  BORNEMANN:  "Uber  den  Buntsandstein  in  Deutschland."     Jena,  1889. 
C.  CLEMENT:  "tiber  die  Bildung  des  Dolomits."     Tscherm.  min.  petr.  Mitt.,  XIV 

(1895),  526. 
G.    R.    CREDNER:  "Die    kristallinischen    Gemengteile    gewisser    Schiefertone    und 

Schiefer."     Zeitschr.  ges.  Naturw.,  Halle,  LXIV  (1874). 

C.  W.   GUMBEL:  "Die  am   Grunde  des   Meeres  vorkommenden   Manganknollen." 

Sitzb.  bayr.  Akad.  Wissensch.,  1878,  189. 

F.  HOPPE-SEYLER:  "Uber  die  Bildung  von  Dolomit."     Zeitschr.  deutsch.  geol.  Ges.t 

XXVII  (1875),  520. 

G.  KLEMM  :  "  Mikroskopische  Untersuchungen  iiber  psammitische  Gesteine."     Ibidem, 

XXXV  (1882),  1. 

E.    KOHLER:  "Uber   die   sog.    Steinsalzziige   des    Salzstocks   von    Berchtesgaden." 
Geogn.  Jahresh.,  XVI  (1903),  105. 

D.  KREICHGAUER:  "Die  Aquatorfrage  in  der  Geologie."     Steyl,  1902. 

L.    LEMIERE:  "Transformation    des   vegetaux    en    combustibles   fossiles."     Compt. 

rend,  VIII  congr.  geol.  intern.,  1900.     Paris,  1901,  502. 
H.   MONKE   UND  F.   BEYSCHLAG:  "Uber  das  Vorkommen  des  Erdols."     Zeitschr. 

prakt.  GeoL,  XIII  (1905),  1,  65,  421. 
J.  MURRAY  ET  A.  F.  RENARD:  "Les  caracteres  microscopiques  des  cendres  volcaniques 

et  des  poussieres  cosmiques  et  leur  role  dans  les  sediments  de  mer  profonde." 

Bull.  mus.  roy.  hist.  nat.  Belgique,  III  (1884),  1. 
Idem:  "Notice  sur  la  classification,  le  mode  de  formation  et  la  distribution  ge"o- 

graphique  des  sediments  de  mer  profonde."     Ibidem,  25. 
C.  OCHSENIUS:  "Die  Bildung  der  Steinsalzlagerstatten  und  ihrer  Mutterlaugensalze." 

Halle,  1877. 

Idem:  "Die  Bildung  machtiger  mariner  Kalkabsatze."     Neues  Jahrb.,  1890,  II,  53. 
Idem:  "Die  Bildung  von  Kohleflozen."     Zeitschr.  deutsch.  geol.  Ges.,  XLIII  (1891),  84. 
Idem:  "Kohle  und  Petroleum."     Zeitschr.  prakt.  GeoL,  1896,  65. 
J.  H.  VAN'T  HOFP:  "Uber  die  Auskristallisation  komplexer  Salzlosungen  bei  konstan- 

ter  Temperatur  unter  Beriicksichtigung  der  natiirlichen  Salzvorkommnisse." 

Zeitschr.  angew.  Chemie,  XIV  (1901),  531. 
J.  H.  VAN'T  HOPP,  W.  MEYERHOPER,  UND  NORM.  SMITH:  "Untersuchungen  liber  die 

Bildungsverhaltnisse  der  ozeanischen  Salzablagerungen,  insbesondere  des  Stass- 

furter  Salzlagers,    XXIII.     Abschluss  und  Zusammenfassung."     Sitzb.  preuss. 

Akad.  Wiss.,  1901,  1034. 

E.  PHILIPPI:  "tJber  Dolomitbildung  und  chemische  Abscheidung  von  Kalk  in  heu- 

tigen  Meeren."     Neues  Jahrb.,  Festband,  1807-1907,  397. 

94 


THE  NATURE  OF  THE  SEDIMENTS  95 

H.    POTONIE:    "Entstehung    der   Steinkohle  und   der   Kaustobiolithe    iiberhaupt." 

5  Aufl.,  Berlin,  1910. 
E.  RAM  ANN:  "Einteilung  und  Benennung  der  Schlammablagerungen."     Monatsber. 

deutsch.  geol  Ges.,  1906,  174. 
JOH.  WALTHER:  "Das  Gesetz  der  Wustenbildung."     Berlin,  1900. 

Composition  of  the  Sediments. — The  materials  forming  the 
sediments  are  derived  both  from  the  weathering  solutions  and 
the  weathered  residues.  If  the  weathered  residues  come  under  the 
influences  of  the  transporting  forces  which  act  at  the  surface  of  the 
earth,  they  are  carried  away  to  form  the  various  mechanical 
sediments,  ceolian  (Aiolos,  God  of  the  winds),  alluvial  (Lat.  alluvius, 
ad,  against,  luere,  to  wash),  or  glacial  (Lat.  glacis,  ice),  depending 
upon  their  mode  of  transportation  by  wind,  moving  waters,  or  ice. 
True  chemical  sediments  are  formed  from  the  weathering  solutions 
by  simple  concentration,  while  organogenic  sediments  originate 
by  the  action  of  organisms  upon  such  solutions. 

Since  sediments  are  derived  from  the  destruction  of  primary 
rocks,  they  are  also  called  secondary  rocks.  There  is  a  marked 
difference  between  the  chemical  compositions  of  igneous  rocks 
and  sediments,  due  to  the  separation  of  the  former  by  chemical 
weathering  into  a  residuum  and  a  solution,  and  the  subsequent 
mechanical  separation  of  the  residuum  into  deposits  of  different- 
sized  grains.  This  difference  is  especially  marked  in  rocks  derived 
from  solutions,  but  it  may  be  clearly  seen,  also,  in  most  seolian 
and  alluvial  sediments.  In  glacial  deposits  it  is  much  less  strongly 
developed,  since  here  chemical  weathering  is  of  slight  importance, 
and  transportation,  in  general,  produces  no  sorting  of  the  material. 
Sediments  may  have  approximately  the  composition  of  igneous 
rocks  where  the  weathered  material  is  deposited  in  slowly  moving 
waters  near  the  original  locality,  or  where  porous  volcanic  ejecta- 
menta  are  deposited  by  so-called  mud-flows.  Such  sediments, 
however,  are  only  of  very  local  significance.  In  general,  the  dif- 
ference is  very  marked,  and  normal  sediments  of  all  groups  differ 
decidedly  in  composition  from  normal  igneous  rocks. 

Mechanical  Sediments. — Mechanical  sediments  are  derived 
from  the  weathered  residues,  therefore  their  compositions,  on  the 
whole,  are  alike.  The  transporting  agents  separate  the  material, 
primarily,  according  to  size  of  grain,  and  upon  this  a  classification 
of  mechanical  sediments  may  be  based.  The  finest  particles  of 
the  weathered  residues,  which  hardly  reach  >{0  mm.,  are  called 


96  FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

dust,  or  if  wet,  mud.  They  form  the  pelites  (Gr.  71-77X6$,  mud). 
Since  they  consist  chiefly  of  the  clay-like  weathered  products 
of  the  feldspars,  they  are  also  called  argillites.  The  grains  of 
coarser  material  or  sand,  may  reach  several  millimeters  in  diameter, 
and  when  solidified  are  called  psammites  (Gr.  ta^os,  sand)  or 
sandstones.  Still  coarser  material  is  called  gravel,  and  this  grades 
into  very  coarse  rubble.  The  few  sediments  composed  predomi- 
nantly of  these  coarse  materials  are  called  psephites  (Gr.  \l/rj<pos, 
a  pebble),  or  conglomerates  (Lat.  conglomerare,  to  heap  together). 

Among  the  constituents  of  the  weathered  residues,  the  colloidal 
alteration  products  of  the  feldspars  are  of  especial  importance. 
These,  together  with  extremely  fine,  sharp,  and  angular  fragments 
of  quartz  and  other  minerals,  compose  the  argillites.  Unweathered 
remnants  of  feldspars,  other  than  microcline,  are  rather  rare. 

Normal  argillites,  in  the  main,  consist  of  the  material  usually  called  clay,  and  an 
examination  by  polarized  light  shows  that  even  very  compact  rocks  consist  pre- 
dominantly of  this  amorphous  substance.  Where  the  first  stages  of  an  inner  molec- 
ular re-arrangement  can  be  recognized,  this  clay-like  substance  shows  a  crystalline, 
micaceous  character.  At  the  same  time  minute  crystals  of  rutile,  so-called  clay- 
slate  needles,  and  small,  but  doubtless  authigenic,  prisms  of  tourmaline,  are  usually 
also  developed.  In  unmetamorphosed  argillites,  neither  clay-slate  needles  nor  the 
mica-like  constituent  of  the  clay  occurs;  and  if,  here  and  there,  tourmaline  appears  in 
such  rocks,  it  is  in  the  form  of  sharp,  angular  fragments,  and  not  in  the  delicate  crystals 
of  the  metamorphosed  rocks. 

The  weathered  residuum,  when  wind  or  water  has  removed  the 
finest  dust,  consists  predominantly  of  quartz  grains,  and  when 
consolidated  it  forms  the  sandstones.  Megascopically  these 
rocks,  essentially  of  rounded  grains,  are  typical  of  all  clastic  rocks. 
Chemically,  by  their  exceedingly  high  silica  content,  they  are 
farther  removed  from  the  composition  of  igneous  rocks  than  the 
argillites. 

Besides  quartz  grains,  sandstones  contain  variable,  usually  rather  small  amounts 
of  a  cementing  material.  In  the  argillaceous  sandstones  this  cement  is  made  up  of 
fine  weathered  material  like  that  which  forms  the  argillites,  and  it  usually  contains  a 
considerable  quantity  of  fine  quartz  splinters,  sharp  and  angular  in  alluvial  sandstones, 
but  showing  in  seolian  formations  a  certain  rounding  of  the  edges  of  even  the  finest 
fragments. 

The  siliceous  cement  of  quartz-sandstones  may  have  originated  in  various  ways. 
Relatively  rarely  does  it  show  the  effects  of  mashing,  and  rarely  are  distinct  organic 
remains  preserved.  It  is  usually  crystalline,  in  some  cases  appearing  as  extremely 
fine-grained  interstitial  matter  between  the  rounded  quartz  grains,  in  others  as  a 
transparent  continuation  of  growth  of  the  more  cloudy,  clastic  quartzes  (Fig.  59). 
This  siliceous  cement  is  generally  the  result  of  diagenesis.  The  mechanical  sediment 
may  have  originally  carried  siliceous  organisms  whose  amorphous  silica  was  dis- 


THE  NATURE  OF  THE  SEDIMENTS  97 

solved  by  the  circulating  water  and  later  crystallized,  or  the  silicification  may  have 
been  produced  by  the  concentration  of  weathering  solutions.  In  other  cases  the  sili- 
ceous cement  has  the  fibrous  appearance  of  chalcedony;  in  still  others  it  consists  of 
opal,  showing  that  the  silica  was  introduced  by  hot  juvenile  waters. 

The  calcareous  cement  of  calcareous  sandstones  is  probably  chiefly  of  organic 
origin,  and  is  derived  from  the  weathering  solutions.  Its  organic  character,  in  some 
cases,  may  still  be  recognized;  in  others  the  mineral  has  recrystallized  and  forms  a 
coarse  aggregate  between  the  rounded  quartz  grains. 

The  marls1  have  a  composite  character,  but  are  predominantly  mechanical 
sediments.  They  consist  of  clayey  and  sandy  material,  and  in  some  transition 
types  contain  a  considerable  admixture  of  organic  materials  and  constituents  of  the 
weathering  solutions,  etc.  While  the  weather-resisting  minerals  of  the  primary  rocks 
are  not  rare  in  argillites  and  sandstones,  they  are  especially  common  in  these  transition 
members.  Thus  the  mica-marls  are  characterized  by  great  amounts  of  muscovite 
and  bleached  biotite,  and  from  them  a  considerable  amount  of  tourmaline,  staurolite, 
garnet,  epidote,  iron-  and  titanium-minerals,  and  chlorite  may  be  separated  by  means 
of  heavy  solutions.  The  loess  deposits  are  somewhat  analogous. 


FIG.  59. — Crystal  sandstone.     Erbach,  Odenwald. 

Finally,  in  all  mechanical  sediments  there  are  found  fresh  crystals  of  apatite,  more 
or  less  cloudy  zircons,  and  very  cloudy  and  altered  crystals  of  monazite  and  xenotime, 
the  latter  having  a  fresh  appearance  only  in  those  kaolin-sandstones  which  were 
deposited  near  the  source  of  the  material.  In  recent  oceanic  deposits  there  occur,  here 
and  there,  very  fine  particles  of  pumice  and  other  constituents  of  volcanic  ash.  These 
were  probably  also  originally  present  in  the  fossiliferous  sediments  but  were  destroyed 
by  the  rock-forming  processes.  Furthermore,  small  particles  of  a  chondrule-like 
formation  which  have  been  found  may  be  meteoritic  dust. 

The  only  constituents  of  the  mechanical  sediments  which  have  crystallized  in 
situ,  other  than  the  quartz  and  calcite  already  mentioned  and  local  occurrences  of 
gypsum  and  rock  salt,  are  a  few  minerals  which  were  modified  and  altered  by  volcanic 
action  during  the  formation  of  the  sediments  or  after  their  consolidation. 

JEolian    Deposits. — ^Eolian    deposits    consist    of    weathered 
material  transported  by  wind.     A  prerequisite  for  their  formation 

1  Marl  is  here  used  in  the  loose  sense  of  calcareous  sediments.     J". 


98 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


is  a  region  devoid  of  vegetation,  such  as  a  seacoast,  an  area  left 
bare  by  the  retreat  of  a  glacier  or  continental  ice,  or  a  desert.  The 
action  of  the  wind  produces  a  complete  sorting  of  the  material 
according  to  size  of  grain.  The  larger  fragments  are  only  slightly 
moved  but  are  more  or  less  corraded  by  the  wind-blown  sands  and 
take  on  triangular  forms.  These  Dreikanter  are  especially 
characteristic  of  the  lowest  horizons  of  desert  deposits. 

The  smaller  sand  grains  are  removed  from  the  coarser  detritus 
during  violent  sand-storms,  and  as  the  force  of  the  wind  decreases, 
a  further  separation  takes  place.  Within  the  desert  itself,  grains 
mostly  of  uniform  size,  are  deposited.  This  desert  sand  is  yellow 
to  red  in  color,  and  in  many  cases  is  heaped  up  to  form  dunes  in 
which  a  bedding  may  be  developed  through  the  varying  force  of 
the  wind  and  the  consequent  deposition  of  coarser  or  finer  material. 
On  the  windward  side,  the  dunes  have  a  slope  of  10°  to  20°,  and 
on  the  lee  side  one  of  approximately  30°.  Where  several  such 


FIG.  60. — Development  of  cross-bedding  by  the  erosion  of  dunes.     (After  J.  Walther.) 

dune  formations  are  separated  by  horizontal  deposits  (Fig.  60), 
they  show  the  cross-  or  diagonal  bedding  which  is  so  characteristic 
of  numerous  seolian  sandstones.  This  is  also  found,  though  much 
less  extensively,  in  fluviatile  deposits,  which  are  confined  to  stream 
courses. 

Dune  sands  show  ripple-marks  (Fig.  61)  at  right  angles  to  the 
direction  of  the  ~7ind.  These  consist  of  small,  wave-like  troughs 
and  crests,  which  differ  from  regular  wave-furrows  of  the  seashore 
primarily  in  having  numerous  ramifications. 

The  finest  dust  and  flaky  and  mica-like  minerals  are  carried 
by  the  wind  far  beyond  the  area  of  the  actual  desert.  They 
eventually  fall  among  the  steppe  grasses,  which  continue  to  grow 
through  the  increasing  depth  of  these  deposits.  Thus  originated 
extensive  and  practically  unstratified  loess  beds,  whose  peculiar 
porous  structure  and  vertical  cleavage  are  to  be  ascribed  to  the 
growth  of  these  grasses.  Massive  deposits  of  loess,  consequently, 
where  cut  by  canyons,  characteristically  show  high,  vertical  walls. 


THE  NATURE  OF  THE  SEDIMENTS  99 

The  fact  that  the  very  finest  quartz-grains  are  usually  distinctly 
rounded  in  the  loess,  is  indicative  of  its  aeolian  origin;  similar  ma- 
terial transported  by  water  would  have  remained  angular. 

Organic  remains  are  rare  in  the  deposits  of  the  desert  itself,  but  may  be  very 
abundant  in  oases;  and  land  snails,  etc.,  are  widely  distributed  in  steppe  deposits. 
Diagenesis,  which  is  absent  from  true  desert  deposition,  causes  a  concentration  of  the 
calcium  content  in  the  loess-kindchen  and  in  the  vertical  root-tubes  of  plants  in  the 
loess. 

Various  salt  deposits  are  formed  by  the  evaporation  of  the  weathering  solutions 
in  deserts  and  steppes,  the  soil  being  in  many  cases  so  thoroughly  saturated  with  salt 
that  hopper-crystals  of  considerable  size  are  developed.  Considerable  amounts  of 
other  salts,  usually  very  impure  through  admixture  with  wind-blown  sand,  also  occur 
in  the  salt-pans.  When  such  deposits  are  solidified  they  are  known  as  salt-pelites, 
and  are  usually  much  brecciated  by  alternating  dessication  and  re-solution  by  sporadic 
rainfalls. 


FIG.  61. — Ripple-marks.     (After  W.  Cross.) 

It  is  worthy  of  note  that  among  the  sediments  formed  immediately  after  the 
retreat  of  the  ice  in  all  glacial  periods  of  considerable  extent,  similar  aeolian  deposits 
occur.  The  reason  for  this  is  that  when  the  anticyclonic  system  of  winds,  which  origi- 
nates over  the  center  of  large  and  much  elevated  continental  ice  areas,  descends  at  the 
periphery  of  the  ice,  it  becomes  warmer  and  moisture  absorbing.  It  is  the  glacial 
Foehn.  Upon  the  retreat  of  the  ice,  the  ground-moraine,  whicL  is  free  from  vegeta- 
tion and  so  porous  that  the  water  sinks  into  it,  is  laid  bare,  and  there  is  thus  developed 
a  true  desert  climate  as  a  result  of  glacial  activity. 

At  the  seashore  the  relationships  are  similar  to  those  in  the  desert.  The  wave 
removes  the  finest  silt  while  the  wind,  during  ebb  tide,  carries  away  the  finest  sand. 
The  latter,  which  is  generally  lighter  in  color  than  desert  sand,  is  carried  far  inland  and 
is  heaped  up  in  shifting  dunes  like  those  of  the*desert. 

Alluvial  Deposits. — Various  mechanical  sediments  are  deposited 
by  moving  water,  and  stream  or  fluviatile  (Lat.  fluvius)  sediments 
are  to  be  separated  from  those  that  are  marine.  On  account  of 
their  mode  of  origin,  the  first  are  local  in  their  occurrence  and  the 
latter  regional. 


100         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

The  fluviatile  deposits  carry,  primarily,  the  coarser  and  the 
specifically  heavier  constituents  of  the  weathered  rocks.  The 
nearer  they  lie  to  the  source,  the  less  they  are  rounded,  the  weaker 
the  force  of  the  water,  the  less  the  separation  into  different  constit- 
uents. With  farther  transportation  the  coarser  detritus  is  first 
rounded,  then  the  finer.  The  very  finest  materials  suffer  no  round- 
ing, even  when  transported  great  distances,  and  in  the  clay-like 
sediments  of  the  ocean  they  are  still  angular.  In  swiftly-flowing 
streams,  the  weathered  products  are  sorted,  and  the  deposits,  in 
spite  of  the  increased  flow  of  water,  are  progressively  finer  from  the 
source  to  the  mouth.  With  decreasing  swiftness,  sand,  and  finally 
mud  is  deposited.  A  stream  which  has  lost  its  transporting  power 
carries  into  the  sea  only  the  finest  particles  which  are  suspended 
in  the  water  and  which  give  to  it  a  cloudy  appearance.  This 
colloidal  material  itself  is  rapidly  precipitated  by  the  catalytic 
action  of  the  salt  of  the  ocean,  and  leads  to  the  formation  of  deltas 
and  the  continental  shelf.  Only  traces  of  the  very  finest  materials 
are  carried  into  the  open  sea,  and  there  the  deposition  of  mechanical 
sediments  is  extremely  slow.  Lakes  along  a  stream  course  act  as 
settling  basins  for  the  coarser  constituents;  the  out-flowing  water, 
therefore,  always  carries  much  finer  material  than  the  in-flowing. 

Fluviatile  deposits  show  some  bedding,  due  to  the  varying  transporting  power  of 
the  stream.  This  carrying  power  increases  during  high  water  and  decreases  during 
dry  periods,  so  that  the  normal  structure  is  not  parallel  but  irregular,  and  shows 
sudden  changes  in  direction.  The  resulting  cross-bedding  is  similar  in  appearance 
to  that  seen  in  sand  dunes. 

Flowing  water  in  many  cases  concentrates  certain  hard  or  heavy  constituents  of 
the  rocks.  Thus  a  pure  quartz-conglomerate  may  be  produced  from  the  detritus  of  a 
granite  which  is  only  here  and  there  cut  by  compact  quartz  dikes,  the  quartz  frag- 
ments being  much  more  resis.tant  than  the  remainder  of  the  decomposed  rock.  Again, 
rich  pockets  of  precious  stones  or  metals,  concentrated  from  rocks  which  in  themselves 
are  poor  in  these  constituents,  may  develop  in  depressions  in  the  underlying  strata. 

Marine  deposits  may  be  divided  into  several  groups.  Of  these 
the  first  is  that  of  the  true  litoral  (Lat.  litus,  coast)  zone  in  which 
the  influence  of  tides,  surf,  and  waves  is  noticeable.  On  precipi- 
tous coasts  the  rocks  are  undercut  by  the  waves  and  boulder 
strands  are  formed.  The  enormous  force  of  the  waves  causes 
abrasion,  and  even  the  largest  blocks  are  rolled  one  over  the  other, 
and  are  rounded  and  fragmented,  while  at  the  same  time  the 
material  is  sorted  according  to  size.  Seaward  from  the  boulder- 
beach  is  one  of  gravel;  then  comes  one  of  sand  which  itself  may 


THE  NATURE  OF  THE 


101 


show  local  accumulations  of  heavy  minerals,  such  as  garnets, 
magnetite,  etc.  Furthermore,  the  waves  carry  away  the  finest 
constituents,  while  during  ebb  tide  when  the  beach  is  exposed,  the 
winds  remove  the  smaller  grains  from  the  washed-out  sand  and 
carry  them  landward  as  clay-free  dune-sand.  The  solidified 
deposits  of  the  literal  zone,  therefore,  are  conglomerates  and 
sandstones,  and  they  may  contain  remnants  of  both  land  and  sea 
organisms.  They  indicate  transgressions  of  the  sea. 

Following  each  other  in  close  succession  from  coast  to  open  sea 
are  transitions  from  gravel,  through  sand,  to  mud  and  ooze.  This 
continental  mud,  in  many  places,  is  of  a  blue  color  on  account  of 
the  iron  sulphide  or  carbonaceous  material  which  it  contains,  and 
it  has  been  named  the  blue-mud. 
In  other  places  it  is  colored  green 
by  glauconite  and  is  then  called 
green-mud.  This  passes  over, 
by  an  increase  in  the  amount 
of  glauconite,  into  true  glauco- 
nite- or  greensand.  Mingled  in 
the  continental  mud  are  the 
cloudy  constituents  of  river 
water  and  the  fine  materials  washed  out  of  the  sandy  beach.  Fine 
grains  of  quartz  are  still  everywhere  present  in  these  deposits, 
and  in  many  cases  they  contain  the  remains  of  plants  and  animals, 
and  beds  of  phytogenic  and  zoogenic  limestone. 

In  true  deep-sea  deposits,  finally,  red  clay  predominates.  This 
is  usually  without  coarse  constituents  but  contains  much  decom- 
posed volcanic  ejectamenta,  and,  in  places,  is  completely  de- 
calcified by  diagenetic  processes.  In  other  places,  however,  these 
deposits  are  replaced  by  zoogenic  or  phytogenic  calcareous  or 
siliceous  oozes.  Sea  deposits  are  generally  distinctly  stratified. 
The  original  bedding  runs  approximately  parallel  to  the  configura- 
tion of  the  underlying  ground,  therefore  many  offshore  deposits 
are  far  from  horizontal  (Fig.  62)  ;  for  example,  sediments  deposited 
against  coral  reefs  are  characteristically  inclined. 

Direct  bedding  originates  through  successive  depositions  of  different  kinds  of 
sediments.  Every  abrupt  change  in  the  nature  of  the  sedimentation  causes  a  sharp 
change  in  the  deposit;  a  gradual  alteration  produces  a  transition  rock,  while  uniform 
deposition  leads  to  homogeneous  sediments.  The  plane  of  separation  between  the 
different  beds,  in  many  cases,  is  marked  by  an  extremely  thin  coating  of  a  different 
character  from  the  rest.  Indirect  bedding  is  produced  when  finer  or  lighter  constit- 


FIG.  62. — Cross-section  through  deposits 
around  the  continental  shelf. 


PRINCIPLES  OF  PETROLOGY 

uents  are  washed  out  of  previously  deposited  material  and  later  sink  down  upon 
the  coarser  particles. 

Glacial  Deposits. — The  chemical  compositions  of  glacial  sedi- 
ments are  nearer  those  of  the  original  rocks  than  is  that  of  any  other 
sediment,  because  they  have  undergone  but  little  chemical  weather- 
ing, and  the  movement  of  the  ice  has  produced  little  sorting  of  the 
material. 

In  glacial  sediments  there  occur  boulders,  in  some  cases  as 
large  as  a  house,  embedded  in  the  finest  till.  This  great  difference 
in  the  size  of  the  components,  and  the  lack  of  bedding,  are  the 


FIG.  63. — Earth  pillars.     Ritten,  Bozen. 

most  marked  characteristics  of  these  deposits,  especially  since  they 
are  the  cause  of  a  peculiar  type  of  erosion  in  formations  which 
have  been  somewhat  hardened  by  diagenetic  processes.  In  such 
cases  the  fine  material  is  not  uniformly  washed  away  by  rain  but 
remains  untouched  where  it  is  protected  by  projecting  covers  of 
larger  blocks,  giving  rise  to  tall  pillars  or  earth-pyramids  (Fig. 
63).  Glacial  deposits  of  different  geologic  periods  are  petro- 
graphically  similar  on  account  of  this  mingling  of  fine  and  coarse 
material  and  the  mingling  of  polished  and  striated  pebbles  of 
the  ground  moraines  with  angular  blocks  from  the  surface  of 
the  ice. 


THE  NATURE  OF  THE  SEDIMENTS  103 

The  occurrence  of  striated  and  scratched  boulders  is  usually  looked  upon  as  a 
criterion  of  glacial  activity.  But  such  striations  may  be  formed  also  by  landslides, 
and  not  infrequently  do  the  brecciated  materials  of  faults  carry  polished  and  scratched 
blocks,  so-called  pseudo-glacial  boulders.  The  extent  of  such  formations,  however,  is 
limited  when  compared  with  true  glacial  deposits. 

Chemical  Sediments. — The  chief  constituents  of  the  weather- 
ing solutions  are  carbonates,  sulphates,  and  chlorides  of  the 
alkalies  and  alkaline  earths,  and  these,  therefore,  are  also  the  chief 
constituents  of  the  chemical  sediments.  Since  these  sediments 
primarily  were  crystallized  from  solutions,  they  generally  have  a 
crystalline  texture,  but  they  may  be  distinguished  from  crystalline 
rocks,  in  the  true  sense,  by  their  constant  content  of  clastic 
constituents. 

The  carbonates  of  the  alkaline  earths,  which  greatly  predomi- 
nate in  normal  fresh  water,  are  soluble  with  difficulty  on  the  one 
hand,  and  are  largely  used  up  by  organisms  on  the  other,  so  that 
they  never  concentrate  in  great  quantity.  Limestones  and 
dolomites,  therefore,  are  of  rather  subordinate  significance  among 
chemical  sediments,  and  the  ocean  is  enriched  in  carbonates  only 
where  carbonated  fresh  water  flows  into  it.  Perhaps  a  direct 
separation  of  the  calcium  carbonate  can  take  place  under  such 
conditions;  very  much  oftener,  however,  it  is  the  result  of  precipi- 
tation caused  by  the  action  of  ammonium  carbonate  from  decom- 
posing organic  matter  upon  the  calcium  sulphate  dissolved  in 
the  water.  At  any  rate,  the  separation  of  calcium  carbonate  by 
living  organisms  is  of  much  greater  importance  in  the  formation 
of  sediments  than  is  its  separation  by  purely  chemical  processes. 

Recent  investigations  have  shown  that  calcium  carbonate 
crystallizes  in  the  form  of  calcite,  usually  as  very  fine  ooze,  in 
water  which  is  but  very  slightly  salt.  Such  a  mode  of  origin  is 
assigned  to  limestones,  such  as  those  at  Solenhofen,  which  are 
exceptionally  compact  and  which  show  hardly  a  trace  of  crystalline 
structure  even  under  the  microscope.  In  very  salt  water,  however, 
the  precipitated  calcium  carbonate  always  takes  the  form  of  arago- 
nite.  This  usually  crystallizes  radially  or  concentrically  around 
minute  foreign  bodies,  such  as  algae,  etc.,  which  were  floating  in 
the  water,  and  forms  the  small  spheres,  rarely  larger  than  pin- 
heads,  which  are  the  chief  constituent  of  oolites. 

Aragonite  formed  in  salt  water  thus  appears  to  be  an  important 
primary  constituent  of  rocks,  yet  sedimentary  rocks  consisting  of 
aragonite  are  very  rare  to  say  the  least,  for  upon  the  subsequent 


104         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

removal  of  the  salt,  the  aragonite,  which  represents  a  labile  state 
of  the  carbonate,  was  re-formed  as  calcite.  At  the  same  time  the 
original  texture  of  the  rock  (cf.  Figs.  5  and  6,  PL  VI)  gave  place 
to  one  which  was  more  crystalline. 

Fresh-water  limestones,  calcareous  sinter,  calcareous  tufas,  stalactites,  etc.,  are 
calcareous  chemical  sediments  precipitated  from  fresh  water  by  means  of  algae,  moss, 
and  the  like,  which  withdraw  the  carbon  dioxide  from  the  dissolved  bicarbonates, 
thereby  causing  the  simple  carbonate,  which  is  not  very  soluble,  to  separate.  Such 
deposits  from  fresh  vadose  water  are  distinguished  with  difficulty  from  others  that  are 
due  to  juvenile  springs,  such  as  the  so-called  Sprudelschale  of  Carlsbad  which  is  of 
considerable  magnitude.  While  these  deposits,  especially  when  from  hot  springs,  also 
contain  aragonite  in  the  incrustations,  they  have  undergone  no  diagenetic  processes 
which  could  have  destroyed  it.  Some  of  these  rocks  from  juvenile  springs  consist 


FIG.  64. — Pea-stone  from  Carlsbad. 

of  crystalline  aragonite-spherules  as  large  as  peas,  and  called  pea-stone  or  pisolite 
(Fig.  64). 

Subordinate  in  amount,  and  probably  derived  exclusively  from  juvenile  springs,  is 
the  siliceous  chemical  sediment,  siliceous  sinter.  This  is  deposited  especially  by 
geysers,  in  many  cases  with  the  aigl  of  certain  siliceous  organisms  which  live  in  hot 
waters.  Here  are  to  be  included  also  many  deposits  from  mineral  springs;  among 
them  the  impregnation  of  sands  with  barytes  and  the  formation  of  baryte-sandstone 
are  of  some  importance. 

By  far  the  most  important  and  widely  distributed  chemical 
sediments,  however,  are  those  that  were  derived  from  the  more 
soluble  constituents  of  the  weathering  solutions,  and  especially 
from  those  portions  which  were  not  used  up  by  organisms,  namely 
sodium  chloride  and  calcium  sulphate. 

Two  very  distinct  types  are  to  be  recognized  in  these  sediments, 
which  occur  in  the  enormous  rock-salt  formations.  The  first  is 
characterized  by  a  great  regularity  in  the  alternation  of  salt  and 


THE  NATURE  OF  THE  SEDIMENTS 


105 


anhydrite  beds,  by  its  extremely  great  horizontal  extent,  and  by 
the  purity  and  often  very  coarse  grain  of  the  salt  crystals.  Further- 
more, in  some  localities,  deposits  of  the  constituents  most  soluble 
in  water,  especially  sulphates  and  chlorides  of  potash  and  mag- 
nesium, are  found  in  the  overlying  beds,  for  example  in  Stassfurt. 
These  soluble  salts  are  completely  wanting  in  the  overlying 
beds  of  the  second  group,  but  on  the  other  hand  they  are  found 
within  the  salt  deposit  itself,  usually  very  subordinate  in  amount 
and  irregular  in  distribution.  The  salt  beds  themselves  show 
no  regularity  in  their  bedding  and  generally  are  not  composed 
of  crystalline  salt  but  of  unlaminated  sandy  clays  strongly  im- 
pregnated with  salt  (Haselgebirge).  The  beds  are  accompanied 
by  anhydrite  and  are  cut  by  innumerable  small  veins  of  pure 
salt  which  show  by  their  occurrence  in  fissures  that  they  are  later 
crystallizations  and  not  original  deposits. 

These  contrasting  characteristics  of  the  two  types  may  also  be  seen  in  recent 
formations.  Where  a  land-locked  bay  is  so  far  cut  off  from  the  open  sea  by  a  bar 
that  the  sea  water  can  enter  it  only  at  flood  tide,  a  strong  concentration  of  the  soluble 


Gypsum 


Rock-salt 


Anhydrite 
with  salt-shale 


Basement 
complex 

FIG.  65. — Formation  of  rock-salt  deposit  with  anhydrite  cover  by  a  flow  from  the 
ocean  in  the  direction  of  the  arrow.     (After  Ochsenius.) 


salts  will  gradually  result.  Calcium  sulphate,  which  is  soluble  with  some  difficulty, 
is  deposited  first  as  gypsum  or  anhydrite  (Fig.  65).  This  is  followed  by  a  deposit 
consisting  chiefly  of  rock-salt.  At  a  later  stage  a  strong  influx  of  water  may  come 
from  the  open  sea,  and  not  only  dilute  the  water  but  bring  in  new  calcium  sulphate 
to  be  precipitated.  The  alternation  of  these  two  deposits  may  recur  in  this  manner 
many  hundreds  of  times,  as  in  the  so-called  banded  salt  of  the  Stassfurt  area. 

The  continued  inflow  and  the  reduced  outflow  of  salt  water  produce  a  concen- 
tration of  the  easily  soluble  salts  until  they  are  finally  precipitated  in  thin,  alternating 
beds.  In  ordinary  salt  deposits,  the  upper  layer  is  a  thick,  insoluble  bed  of  anhydrite, 
the  so-called  anhydrite-hat,  but  in  a  gradually  drying  up  bay  the  materials  for  such  a 
bed  are  wanting  and  the  easily-soluble  salts  already  deposited  can  be  protected  from 
the  rain  only  where  great  masses  of  dust  from,the  neighboring  land  are  blown  into  the 
dried-up  basin  and  form  an  impervious  cover  of  silt  over  them.  This  first  group  of 
rock-salt  deposits  may  be  called  the  marine  type. 

The  second  group  of  deposits  is  analogous  to  those  of  the  salt-pans  of  the  deserts. 
The  concentrated  salt  water,  mixed  with  the  dust  which  is  blown  in,  dried  up  without 


106         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

lamination  in  the  form  of  salt-pelite.  During  the  process  of  drying,  the  deposit  becomes 
filled  with  cracks,  and  in  these  the  material  which  is  leached  out  by  the  occasional 
rainfalls  is  again  deposited.  Since  the  mud  dries  up  completely,  no  separation  takes 
place  between  the  constituents  which  are  soluble  with  ease  and  those  which  are  soluble 
with  difficulty,  consequently  when  the  desert  streams  bring  in  water  which  is  already 
rich  in  salt,  the  process  is  repeated  over  and  over.  Contrasting  with  the  uniform 
conditions  of  deposition  of  the  normal  facies,  therefore,  are  the  very  irregular  condi- 
tions of  the  second.  The  latter,  furthermore,  is  of  very  local  importance.  Most 
of  the  Alpine  deposits  of  rock-salt  belong  to  this  class,  and  they  are  therefore  spoken 
of  as  the  Alpine  or  desert  type. 

It  may  be  remarked,  further,  that  on  account  of  the  easy  solubility  of  the  rock- 
salt  formation  on  the  one  hand,  and  the  great  increase  in  volume  produced  by  the 
alteration  of  anhydrite  to  gypsum  on  the  other,  great  dislocations  are  common  in  these 
deposits,  and  their  original  forms  are  recognized  with  difficulty  in  many  cases.  Rock- 
salt,  like  anhydrite,  occurs  only  in  semi-arid  regions,  and  cliffs  of  rock-salt  are  seen  only 
in  steppes  and  deserts. 

Organogenic  Sediments. — As  already  mentioned,  organogenic 
(Gr.  opyavovy  organ,  yeveais,  origin)  sediments  or  bioliths  (Gr.  /3ios, 
life)  may  be  divided  into  two  groups,  one  consisting  predominantly 
of  the  inorganic  skeletal  parts  of  organisms  and  forming  carbon- 
aceous, siliceous,  or  phosphoritic  rocks,  the  other,  or  caustobio- 
liihic  (Gr.  KCWO-TIKOS,  burning)  group,  consisting  predominantly  of 
organic  material  such  as  coal,  bitumen,  and  petroleum. 

Among  sediments,  the  calcareous  organogenic  sediments,  in 
particular  zoogenic  (Gr.  ^wo*/,  animal)  limestones  derived  from  animal 
remains,  are  of  especial  importance  in  all  geologic  formations,  and 
even  at  the  present  time  such  deposits  are  being  extensively  formed. 
Calcareous  sediments,  in  part,  have  the  form  of  normal  bedded 
rocks.  Here  belong,  for  example,  shell  sands  which  are  made  up 
predominantly  of  angular  fragments  of  shells,  echinoderm  breccias, 
etc.;  all  of  them  usually  formed  in  shallow  seas.  Other  calcite 
secreting  animals  are  plankton  (Gr.  ir\avKTbv,  wandering),  and  their 
remains,  in  part,  are  mingled  with  the  mechanical  sediments  of 
the  different  sea-zones,  in  part  form  pure  deposits,  such  as  globi- 
gerina  and  pteropod  ooze.  The  latter  two  consist  predominantly 
of  calcareous  material.  In  the  deepest  portions  of  the  ocean,  the 
true  deep  sea,  such  deposits  are  comparatively  rare,  for  the  water 
of  the  deeps,  being  rich  in  carbon  dioxide,  soon  dissolves  the 
calcareous  material. 

In  other  places  calcium  carbonate  is  deposited  as  oyster  and 
mussel  banks,  and  as  reefs  by  corals,  calcareous  sponges,  bryozoa, 
and  serpulites.  The  individual  reefs,  especially  those  of  coral, 
may  be  of  great  size.  They  form  fringing  or  barrier  reefs  in  the 


THE  NATURE  OF  THE  SEDIMENTS 


107 


tropical  seas,  and  completely  encircle  the  coasts  except  for  certain 
openings  where  the  inflow  of  water  from  streams  has  made  the  sea 
less  salt.  In  other  cases  they  form  the  so-called  lagoon  reefs  or 
atolls,  the  typical  form  of  coral  islands. 


Island  Reef  materials 

FIG.  66. — Growth  of  a  coral  reef  by  subsidence,     a  barrier  reef,  b  fringing  reef, 

c  atoll  reef. 

Although  these  organisms  are  able  to  live  only  in  shallow  water, 
coral  is  found  to  depths  of  thousands  of  meters,  the  slow  sinking  of 
the  ocean  bottom  continually  renewing  the  conditions  for  their 
further  growth  (Fig.  66).  In  many  cases  the  reefs  have  extremely 


••v  v 

FIG.  67. — Reef  passing  into  normal  bedded  deposits  through  a  zone  of  brecciated 
material.     (After  E.  Fraas.) 

steep  walls  and  pass  over  into  normal  bedded  sediments  through  a 
zone  of  brecciated  material  (Fig.  67). 

Phytogenic  (Gr.  <pvrov,  plant)  calcareous  deposits  are  formed  in  the  sea  by  the  action 
of  algse,  especially  Lithothamnion,  Gyroporella,  etc.,  and  may  be  quite  extensive. 
It  is  noteworthy  that  magnesium  carbonate  is  more  abundant  in  these  formations 
than  in  zoogenic  rocks,  although  it  forms  but  a  small  portion  of  any  organic  skeletons 
except  those  of  calcareous  algse,  in  which  it  may  reach  15  per  cent. 


108         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

Organogenic  siliceous  sediments  are  in  part  zoogenic  and  in  part  phytogenic,  the 
former  chiefly  composed  of  radiolaria,  more  rarely  of  siliceous  sponges.  It  is  signifi- 
cant that  siliceous  muds  occur  even  in  the  deepest  parts  of  the  ocean,  and  locally 
form  considerable  deposits.  The  siliceous  schists  and  radiolarian  hornstones, 
derived  from  the  siliceous  muds  of  former  geologic  periods,  are  usually  completely 
recrystallized,  the  organic  silica  being  very  easily  soluble.  In  more  recent  deposits 
of  polishing  slate,  kieselgur,  etc.,  on  the  other  hand,  amorphous  silica  remains. 

Finally,  some  of  the  phosphorites  belong  to  the  organogenic  deposits.  They 
consist  of  aggregates  of  bones  of  mammals  and  of  guano  and  coprolites,  or  of  lime- 
stones which  have  been  acted  upon  by  such  material. 

The  second  group  of  organogenic  rocks  consists  of  those  which 
are  composed  predominantly  of  carbonaceous  material.  Their 
mode  of  origin  is,  to  a  great  extent,  still  unexplained.  Even  in 
regard  to  the  coals,  which  form  the  principal  part  of  the  most 
important  group,  very  little  is  known.  They  are,  without  doubt, 
predominantly  of  phytogenic  origin,  and  represent,  in  the  main, 
the  remnants  of  a  luxuriant,  primitive  vegetation.  Their  organic 
structure  is  still  distinctly  recognizable  in  innumerable  occur- 
rences, especially  in  microscopical  preparations  from  which  the 
opaque  carbonaceous  substances  have  been  removed  by  potassium 
chlorate  and  nitric  acid. 

Coal  characteristically  occurs  in  beds  of  rather  uniform  thick- 
ness, and  even  very  thin  beds  may  extend  over  great  areas;  the 
great  Appalachian  field,  for  example,  covering  over  135,000 
sq.  km.  Although  coal  occurs  in  all  geologic  periods,  it  was 
not  deposited  in  equal  abundance  in  all  of  them,  two  only  being 
marked  by  an  especial  abundance,  namely  the  Carboniferous 
and  the  Tertiary.  Extensive  recent  formations  are  unknown. 

The  characters  of  the  coals  of  different  formations  are  not  the 
same.  In  general,  the  oldest  coals  are  richest  in  carbon,  conse- 
quently it  was  once  thought  that  the  process  of  altering  wood  to  coal 
was  an  extremely  slow  one,  requiring  geologic  periods  for  its 
completion.  On  the  other  hand  it  has  been  observed  that  coal 
beds  have  a  higher  carbon  content  where  they  are  cut  by  faults, 
and  that  in  strongly  folded  mountain  regions  they  are  richer  than 
deposits  of  the  same  age  in  undisturbed  strata.  The  increase  in 
the  carbon  content,  therefore,  has  been  ascribed  to  mountain- 
pressure,  but  it  is  difficult  to  determine  how  far  this  hypothesis 
is  correct.  While  the  change  in  the  coal  in  strongly  disturbed  re- 
gions is  not  to  be  questioned,  it  is  more  probable  that  in  general 
the  present  character  of  the  coal  is  due  to  the  diagenetic  processes 
of  decay. 


THE  NATURE  OF  THE  SEDIMENTS  109 

There  is  a  characteristic  difference  in  the  mode  of  formation 
of  bituminous  coal  and  of  the  usually  much  less  extensive  brown- 
coal.  The  former,  in  many  cases,  occurs  in  beds  which  are  repeated 
a  great  number  of  times,  one  above  the  other.  At  Aix  la  Chapelle 
45  may  be  counted,  at  Mons  110,  and  in  the  Donez  basin,  225. 
The  number  of  brown-coal  beds  is  usually  very  limited,  and  rarely 
exceeds  six.  The  thickness  of  the  individual  beds,  however,  is 
inversely  proportional  to  their  number.  Workable  bituminous 
beds  average  30  to  125  cm.  in  thickness,  while  beds  of  10  meters 


FIG.  68. — Petrified  forest  of  Treuil,  near  St.  Etienne.     Dept.  Loire. 

are  rare.     On  the  other  hand,  brown-coal  beds  15  to  30  meters  in 
thickness  are  common. 

Two  methods  of  coal  deposition  have  been  suggested.  The  material  may  have 
accumulated  where  the  coal  plants  grew,  or  it  may  have  been  transported.  In  the 
former  case  the  fallen  trees  of  a  heavily  timbered,  marshy,  primeval  forest  were  trans- 
formed into  coal  under  water,  and  as  the  deposit  increased  more  and  more  in  thickness, 
it  even  enclosed  the  still-standing  trunks.  As  a  consequence,  fossil  forests  (Fig.  68), 
still  upright,  are  found  in  various  coal  deposits,  and  in  the  underlying  beds  there  are 
great  rooted  stumps  or  stigmaria.  On  this  theory  it  is  extremely  difficult  to  explain 
how  so  many  coal  beds  could  have  attained  such  great  thicknesses.  Such  abundant 
vegetation  as  is  necessary  for  the  formation  of  coal  could  hardly  have  continued  to 
grow  in  the  underlying,  half-carbonized  masses,  which  must  have  been  many  meters 
in  thickness  and  extremely  poor  in  the  salts  necessary  for  plant  growth. 

According  to  the  other  theory,  coal  beds  are  secondary.     They  are  supposed  to 


110         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

have  been  deposited  by  the  action  of  streams  which  flowed  through  the  primeval  forests 
and  carried  great  quantities  of  wood  to  the  sea.  There,  possibly,  the  logs  formed 
dams  of  such  a  character  that  they  permitted  the  fine  detritus  to  pass  but  held  back 
bushes  and  tree-trunks  carried  in  by  the  stream.  As  these  became  saturated  with 
water,  they  sank,  and  were  later  altered  to  coal.  It  is  conceivable  that  trees  with 
crowns  and  roots,  torn  bodily  from  the  mainland,  might  sink  in  vertical  positions 
with  their  heavy  roots  downward,  and  thus,  when  wedged  in  by  later  deposits,  give 
the  impression  of  trees  in  place.  Subsequently,  during  some  particularly  high  stage 
of  the  water,  the  bar  may  have  been  torn  away,  and  the  entire  deposit  flooded  with 
detritus.  For  a  long  time  thereafter  the  floating  timber  may  have  been  carried  away, 
while  the  materials  for  conglomerates,  sandstones,  and  shales  were  deposited  upon 
the  carbonizing  stratum  of  wood.  Eventually  another  bar  formed,  and  the  deposition 
of  wood  was  renewed.  An  objection  to  this  theory  is  the  great  horizontal  extent  of 
some  of  the  coal  deposits,  which  is  out  of  all  proportion  to  the  size  of  the  continent 


m 


M 
Drill  holes 


Oil  sand  Drill  holes  Sediments 

FIG.  69. — Petroleum  horizon  showing  relation  of  the  oil  line  to  the  anticline. 

during  the  Carboniferous  period.     The  problem  of  the  origin,  therefore,  cannot  be 
considered  as  definitely  solved  at  the  present  time. 

The  two  principal  coal-measures  were  formed  immediately  after  the  two  periods  of 
greatest  volcanic  activity  recorded  in  geologic  history.  This  may  be  explained, 
perhaps,  if  it  is  assumed  that  the  great  amount  of  carbon  dioxide  poured  out  into  the 
atmosphere  by  volcanic  activity  greatly  stimulated  plant  growth  and  resulted  in  the 
deposition  of  the  coal. 

Finally  the  formation  of  bitumen  and  petroleum  are  to  be  con- 
sidered. The  relationships  between  the  two  substances  are  not  yet 
very  clear.  One  view  is,  that  bitumen,  in  the  main,  is  a  residual 
product  of  petroleum;  another,  conversely,  that  petroleum  springs 
are  due  to  the  distillation  of  bituminous  schists.  The  conditions 
of  formation  of  petroleum  itself  are  not  yet  perfectly  understood, 


THE  NATURE  OF  THE  SEDIMENTS  111 

and  it  is  difficult  to  settle  the  question  geologically,  since  the  oil 
continually  changes  its  position  within  the  earth  and  is  probably 
never  found  in  the  place  where  it  was  formed.  The  geologic 
relationships  of  petroleum  accumulations  show  certain  peculiari- 
ties which  make  a  genetic  explanation  no  simple  matter.  The 
oil  appears  exclusively  in  the  outer  zones  of  folded  regions,  and 
the  principal  wells  occur  along  so-called  oil-lines.  These  corre- 
spond to  the  crests  of  anticlines,  which  are  enriched  by  the  tendency 
of  the  oil  to  rise  (Fig.  69). 

The  association  of  petroleum  with  great  folds  of  the  earth's  crust,  and  the  great 
quantity  of  oil  occurring  in  certain  regions  are  the  chief  objections  to  the  theory  of  its 
organic  origin.  If  this  theory  is  correct,  the  oil  must  have  originated  by  the  decay  of 
animals  or  plants  rich  in  fat,  yet  great  accumulations  of  organic  materials  are  but  rarely 
found.  It  is  true  that  bituminous  shales  are  rather  widely  distributed,  but  they  are 
generally  not  very  thick,  and  their  bituminous  content  is  too  small  to  be  the  source 
of  the  overlying  petroleum.  Certain  provinces,  for  example,  have  produced  great 
quantities  of  oil  uninterruptedly  for  hundreds  of  years.  Since  no  known  sediments 
could  by  any  possibility  have  produced  so  much  oil,  Humboldt  and  Mendelejew  con- 
cluded that  it  must  be  of  inorganic  origin.  They  assumed  tljat  great  masses  of 
carbides  and  other  similar  compounds  in  the  interior  of  the  earth  were  decomposed  by 
circulating  water,  and  that  the  resulting  hydrocarbons  rose  as  petroleum  in  greatly 
folded  strata. 

The  above  objections  to  the  organic  theory  would  seem  favorable  to  the  inorganic, 
yet  the  foundations  upon  which  the  latter  is  based  are  altogether  too  hypothetical. 
While  very  little  is  known  as  to  the  earth's  interior,  we  have  no  right  to  assume  that 
carbides  exist  at  depths  which  can  be  reached  by  circulating  waters,  even  though  car- 
bon compounds  are  present  in  great  abundance  within  the  earth.  Furthermore,  the 
invariable  association  of  petroleum  with  rock-salt,  which  is  not  explained  by  this 
theory,  seems  to  be  a  strong  proof  for  its  organic  origin.  On  the  whole,  the  organic 
origin  seems  to  be  the  more  probable. 

Diagenesis. — A  comparison  of  recent  formations  with  those  of 
past  geologic  times  shows  that  the  rock-forming  processes  usually 
do  not  end  with  deposition,  for  there  are  characteristic  differences 
which  must  have  originated  in  subsequent  periods.  These  altera- 
tions, caused  by  the  circulating  surface  waters,  begin  to  take  place 
at  the  moment  of  deposition  of  a  sediment,  and  continue  until 
that  stratum  is  withdrawn  from  their  zone  of  action.  These  phys- 
ical and  chemical  alterations,  which  may  be  described  by  the  term 
diagenesis  (Gr.  biaylyvo^ai,  to  continue),  are  to  be  distinguished  from 
the  changes  produced  by  metamorphism.  Under  the  latter  term 
are  included  those  later  alterations  which  took  place  in  the  rocks 
after  their  withdrawal  from  the  sphere  of  action  of  the  superficial 
agents  and  after  they  had  already  become  geologic  bodies. 


112         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

The  phenomena  of  metamorphism,  which  are  described  in  a  later  chapter,  are 
usually  much  more  intense  than  those  of  diagenesis.  The  intensity  of  the  latter  varies 
considerably  in  different  deposits.  Since  it  depends  primarily  upon  aqueous  solutions, 
it  is  especially  intense  in  deep-sea  formations,  and  is  only  well  developed  in  steppe  and 
desert  deposits  when  these  come  under  the  influence  of  a  more  humid  climate. 

Diagenesis  in  some  cases  consists  of  the  solution  of  certain  constituents  of  clastic 
sediments  by  the  water  standing  above  them,  as  in  the  case  of  ocean  sediments, 
which  are  thus  usually  entirely  free  from  sodium  chloride,  or  by  the  leaching  of  the 
soluble  salts  by  ground-water  as  in  the  case  of  alluvial  sediments.  In  a  similar  manner, 
the  water  of  the  deep  sea  produces,  by  the  great  pressures  there  existing,  a  de-calcifica- 
tion of  the  sediments,  so  that  true  deep-sea  deposits  may  be  entirely  free  from  calcium. 
Other  diagenetic  processes  produce  a  concentration  of  certain  constituents;  for  ex- 
ample, of  manganese  iron  oxides  in  almost  all  recent  sea-deposits,  of  calcite  in  clays  as 
in  septaria,  of  phosphorite  and  heavy  spar,  of  pyrite  and  marcasite,  of  aluminous 


FIG.  70. — Thin  section  of  a  bone  showing  the  complete  preservation  of  the  texture. 
From  the  Wichita  beds  (Permian)  of  Texas. 

spharosiderite,  and  of  flint  and  hornstone.  Here  also  belong  silicification  and  minerali- 
zation of  organic  remains. 

Other  diagenetic  processes  lead  to  the  formation  of  new  minerals  such  as  glauconite, 
which  usually  forms  grains  in  foraminifera  tests,  and  which  may  be  so  abundant  that 
it  forms  true  greensand.  Further,  zeolites  may  be  formed  locally  in  sea-sediments  in 
regions  of  volcanic  activity,  and  new  formations  are  especially  abundant  where 
mineral  springs  saturate  deposited  detritus. 

At  present  the  extent  of  induration  of  argillites  as  a  result  of  diagenesis  cannot  be 
stated;  recent  sea-deposits,  certainly,  are  still  slimy  and  plastic  to  a  considerable  depth. 
The  cementation  of  limestone  fragments  and  sand  by  calcium  carbonate,  and  of 
sandstones  by  silica,  however,  are  in  many  cases  due  to  simple  diagenesis. 

The  normal  fossilization  of  organic  skeletons  is  also  due  to  simple  diagenesis. 
Calcareous  skeletons,  which  originally  consisted  of  chitin-like  material  intergrown 
with  conchite  or  calcite,  are  of  great  importance  in  the  formation  of  rocks.  In  the 
process  of  fossilization,  the  organic  substances  are  first  destroyed,  either  by  complete 
removal,  or  by  alteration  to  carbonaceous  or  bituminous  substances.  At  the  same 
time,  the  fine  structural  details  of  the  inorganic  parts  are  destroyed  more  and  more 
by  recrystallization,  which  always  takes  place  when  the  material  was  originally  calcite, 
and  in  some  cases  when  it  was  conchite.  In  many  cases  the  inorganic  parts  of  the 


THE  NATURE  OF  THE  SEDIMENTS  113 

skeleton  are  carried  away  with  the  organic  and  only  impressions  or  casts  remain,  or 
the  original  material  is  replaced  by  a  foreign  substance  such  as  pyrite  or  hornstone,  or, 
in  the  case  of  foraminifera,  by  glauconite.  Skeletons  like  those  of  echinoderms  or 
calcareous  sponges,  which  consist  of  but  few  calcite  crystals,  are  best  able  to  withstand 
these  and  all  other  alteration  processes. 

Siliceous  skeletons,  which  originally  consisted  of  glass-like  amorphous  silica,  are 
especially  likely  to  break  up  under  the  processes  of  diagenesis.  They  may  simply 
alter  to  crystalline  aggregates  of  quartz  with  only  rare  traces  of  the  original  organic 
structure,  or  be  carried  away  completely  by  water  to  accumulate  elsewhere  as  con- 
cretions of  hornstone  and  flint,  or  recrystallize  as  quartz  in  fissures  in  the  rocks.  In 
many  cases  the  spaces  formerly  occupied  by  the  leached  silica  are  filled  by  calcite. 

The  bones  of  mammals  appear  to  be  able  to  resist  the  processes  of  diagenesis  to  a 
great  extent,  so  that  in  many  cases  they  preserve  their  original  texture  when  com- 
pletely fossilized  (Fig.  70). 

Dolomitization,  which  is  undoubtedly  a  diagenetic  process, 
consists  in  the  addition  of  magnesium  carbonate  to  organic  lime- 
stone deposits  which  were  originally  free  from,  or  poor  in,  magnesia. 
Innumerable  synthetic  experiments  under  conditions  similar  to 
those  in  nature  have  shown  that  calcite  cannot  be  altered  to  dolo- 
mite, while,  on  the  other  hand,  aragonite  and  conchite  take  up  mag- 
nesia quite  readily.  For  this  reason,  apparently,  dolomite  is  found 
primarily  in  deposits  whose  calcareous  parts  originally  consisted 
of  these  unstable  forms  of  calcium  carbonate.  Patches  of  such 
dolomitic  alterations  in  recent  coral  reefs  find  an  explanation  in 
this  mode  of  origin.  There  is  usually  a  considerable  alteration  of 
the  texture  of  the  rock  accompanying  dolomitization,  and  in  place 
of  the  original  compact  limestone,  there  develops  a  porous  rock 
filled  with  drusy  cavities.  Further,  the  dolomites  are  distinctly 
crystalline,  and  the  organic  remains  may  be  entirely  obliterated 
or  only  preserved  as  casts. 

Although  the  general  process  of  dolomitization  is  rather  well  understood,  the  special 
causes  for  these  local  diagenetic  alterations  are  not  known,  and  the  hypothesis  of 
J.  Walther  that  fission  algae  produce  chemical  alterations,  is  not  to  be  discarded  lightly. 
The  change  from  calcite  to  dolomite  is  produced  by  the  magnesia  in  sea  water,  which 
is  especially  active  when  greatly  concentrated  and  warmed,  as  in  tropical  lagoons. 
The  alteration  proceeds  irregularly,  and  dolomite,  therefore,  is  characterized  by  a  vari- 
able composition.  It  is  noteworthy,  further,  that  only  enough  magnesia  is  used  by 
the  diagenetic  process  to  form  normal  dolomite.  Magnesia-rich  rocks,  which  are  tran- 
sitions to  magnesites,  are  formed  exclusively  by  thermal  processes,  and  therefore  are 
generally  associated  with  volcanic  rocks. 

A  final  form  of  diagenesis  is  that  of  organic  substances.  Coal  is 
formed  by  the  decay  of  cellulose  under  water.  The  difference 
between  anthracite,  bituminous,  and  brown-coal  may  be  primary, 
in  so  far  as  it  depends  upon  differences  in  the  original  carboniza- 


114         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

tion  processes,  that  is,  upon  diagenesis  itself,  or  secondary  and  be 
due  to  the  slow  continuation  of  its  formation  during  succeeding 
geologic  periods. 

Diagenesis  of  organic  substances  is  due  to  fermentation.  According  to  the 
theory  of  Lemiere,  three  chief  agencies  are  involved ;  living  ferments  which  cause  true 
carbonization,  soluble  ferments  which  produce  masceration,  and  the  resulting  toxic 
substances  which  nullify  the  action  of  the  other  agents  and  terminate  the  carboniza- 
tion process.  No  changes  are  produced  in  the  coal  after  the  cessation  of  the  action 
of  the  ferments,  so  that  brown-coal,  thereafter,  can  never  become  bituminous.  Like- 
wise anthracite  became  anthracite  shortly  after  its  deposition.  In  the  oldest  coal 
periods  the  living  ferments  predominated,  while  during  the  formation  of  bituminous 
coal  the  soluble  ferments  were  in  excess. 

According  to  other  views  only  the  beginning  of  carbonization  is  a  true  diagenetic 
process,  and  the  enrichment  in  carbon  will  continue  even  in  the  oldest  deposits  until 
they  are  converted  to  a  final  state  of  pure  carbon.  It  is  difficult  to  understand  why 
this  process  should  continue  so  long,  but  as  already  mentioned,  cases  are  known 
where  later  dislocations,  etc.,  have  increased  the  carbon  contents  of  certain  parts  of 
coal  deposits. 

The  theory  of  the  organic  origin  of  petroleum  seems  to  explain  the  existing  associa- 
tion of  petroleum  with  rock-salt,  for  it  has  been  shown  experimentally  that  the  decay 
of  animal,  and  perhaps  also  of  plant,  remains  under  a  cover  of  salt  water  will  produce 
petroleum-like  substances. 

Recent  and  Fossil  Sediments. — The  petrologic  study  of  the 
characteristics  of  sediments,  and  a  comparison  of  equivalent 
formations  in  different  parts  of  the  earth,  have  led  to  many  sur- 
prising results.  A  characteristic  of  sediments  now  being  formed 
in  the  tropics  is  their  red  color,  due  to  the  color  of  the  weathered 
products.  The  processes  of  weathering,  during  earlier  geologic 
periods,  were  undoubtedly  the  same  as  those  operating  at  the 
present  time,  and  the  occurrence  of  red  sandstones,  red  clays,  and 
red  calcareous  shales  is  probably  just  as  legitimate  a  proof  of  a 
former  tropical  climate  as  is  the  presence  of  coral  reefs.  Further- 
more, extensive  deposits  of  homogeneous,  fossil-free  sandstones 
indicate  a  former  desert  climate  just  as  certainly  as  heterogeneous 
conglomerates  of  striated  boulders  indicate  glaciation. 

Thus  a  comparative  petrographic  examination  of  the  facies  of  different  formations 
shows  not  only  the  boundaries  between  the  mainland  and  ocean  basins  of  former 
geologic  periods,  but  indicates  the  climatic  conditions  through  which  the  earth  has 
passed.  While  the  mean  climate  of  the  earth  has  probably  undergone  no  noteworthy 
change  since  the  oldest  fossil-bearing  formations  were  deposited,  the  notable  differ- 
ences in  the  petrographic  characters  of  the  sediments  seem  to  indicate  that  during 
geologic  periods  there  have  been  geographic  displacements  in  relation  to  climatic 
conditions,  displacements  such  as  may  have  been  produced  by  a  change  in  the  position 
of  the  equator  or  by  other  astronomic  phenomena. 


THE  NATURE  OF  THE  SEDIMENTS  115 

Recent  marine  sediments  generally  differ  from  older  deposits 
in  being  plastic  instead  of  more  or  less  consolidated,  that  is,  the 
latter  are  better  cemented.  Occasional  exceptions  to  this  rule,  such 
as  the  occurrence  of  sands  and  clays  in  the  almost  undisturbed 
Cambrian  deposits  of  the  Baltic  Sea  province,  or  of  hard,  trans- 
versely schistose  slates  in  the  Oligocene  of  the  Glarner  Alps,  simply 
indicate  that  these  differences  are  not  diagenetic  but  that  they 
originated  some  time  after  the  deposition. 

Pressure  has  been  considered,  apparently  not  incorrectly,  the  primary  cause  for 
the  consolidation.  In  general,  strongly-folded  argillites  with  transverse  cleavage 
are  more  consolidated  than  are  those  which  have  not  been  subjected  to  pressure. 
The  fundamental  cause  for  this  is  not  definitely  known;  in  the  main  it  appears  to  be 
due  to  a  decrease  in  porosity,  as  is  the  case  when  loose,  artificial  aggregates  are  com- 
pressed to  coherent  masses  by  great  hydraulic  pressure.  At  any  rate,  the  character  of 
the  resulting  alteration  differs  greatly  from  that  produced  by  dynamometamorphism, 
for  here  there  is  no  question  of  recrystallization  or  of  other  molecular  re-arrange- 
ments. The  rocks  preserve  their  original  clastic  character,  and  even  the  colloidal, 
clay-like  substances  in  very  compact  roofing-slates  are  unaltered. 

The  re-formation  of  the  sedimentary  rocks  after  the  ending  of  true  diagenetic  proc- 
esses has  been  too  little  studied  to  be  conclusively  treated  at  present.  No  definite 
conclusions  can  be  drawn  as  to  the  active  agents  aside  from  than  those  that  are  of 
volcanic  origin,  nor  are  the  processes  themselves  clearly  understood. 


VII.  CONTACT-METAMORPHISM 

LITERATURE 

CH.  BARROIS:  "Recherches  sur  les  terrains  anciens  des  Asturies  et  de  la  Galice." 

Lille,  1882. 
Idem:  "Memoire  sur  les  gres  metamorphiques  du  massif  granitique  de  Gueme'neV' 

Ann.  soc.  geol  Nord.,  XI  (1884),  103. 
Idem :  "  Memoire  sur  le  granite  de  Rostrenen,  ses  apophyses  et  ses  contacts."     Ibidem, 

XI  (1885),  1. 
C.  W.  BROGGER:  "Die  silurischen  Etagen  2  und  3  im  Kristianiagebiet  und  auf  Eker." 

Univ.  Programm.  Kristiania,  1882. 
A.  DAUB  REE:  "Etude  et  experiences  synthe'tiques  sur  le  metamorphisme  et  sur  la 

formation  des  roches  cristallines."     Mem.  pres.  a  I'acad.     Paris,  XVII  (1860). 
V.  M.  GOLDSCHMIDT:  "Die  Gesetze  der  Gesteinsmetamorphose  mit  Beispielen  aus 

der  Geologic  des  siidlichen  Norwegen."  Vid.  selsk.  shrift,  1,  Kl.,  1912,  Nr.  22. 
G:  KLEMM:  "Beitrage  zur  Kenntnis  des  kristallinischen  Grundgebirges  im  Spessart." 

Abhandl.  hess.  geol.  Landesanst.     1895,  II,  Heft  4. 
^  A.  LACROIX:  "Considerations  sur  le  metamorphisme  de  contact,  auxquelles  conduit 

l'e"tude  des  phe"nomenes  de  contact  de  la  Iherzolithe  des  Pyrenees."     Compt. 

Rend.,  CXX  (1895),  388. 
,    J.  LEMBERG:  "tJber  Gesteinsumwandlungen  bei  Predazzo  und  am  Monzoni."     Zeit- 

schr.  deutsch.  geol.  Ges.,  XXIX  (1877),  457. 
K.  A.  LOSSEN:  "  Metamorphische  Schichten  aus  der  palaozoischen  Schichtenfolge 

des  Ostharzes."     Zeitschr.  deutsch.  geol.  Ges.,  XXI  (1869),  281. 
Idem:  "Uber  den  Spilosit  und  Desmosit."     Ibidem,  XXIV  (1872),  701. 
'   H.  ROSENBUSCH:  "Die  Steiger  Schiefer."     Abhandl.  geol.  Spes.  Karte  Elsass  Lothr., 

1877,  I,  79. 
i/E.  WEINSCHENK:  "tjber  Serpentine  aus  den  ostlichen  Zentralalpen  und  deren  Kon- 

taktbildungen."     Munchen,  1891. 
Idem:  "Die  Kieslagerstatte  im  Silberberg  bei  Bodenmais."     Abhandl.  bayer.  Akad. 

Wiss,  II  Klasse,  XXI  (1901),  (II),  351. 
Idem:  "Beitrage  zur  Petrographie  der  ostlichen  Zentralalpen.  Ill :  Die  kontakmeta- 

morphe    Schieferhiille  und    ihre    Bedeutung    fur    die  Lehre  vom  allgemeinen 

Metamorphismus."     Ibidem,  XXII  (1903),  II,  263. 
*   GEO.  H.  WILLIAMS:  "Contributions  to  the  Geology  of  the  Cortlandt  Series  near 

Peekskill."     Amer.  Jour.  Sci.,  XXVI  (1888),  254. 
F.  ZIRKEL:  "Beitrage  zur  geologischen  Kenntnis  der  Pyrenaen."     Zeitschr.  deutsch. 

geol.  Ges.,  XIX  (1867),  68. 

Agents  of  Contact  -me  tamorphism. — Different  magmas  rising 
from  the  interior  of  the  earth  contain  different  amounts  of  dis- 
solved gases  and  vapors.  As  was  shown  above,  these  gases  act 
as  mineralizers  during  the  solidification  of  the  rocks,  and  in  many 
cases,  with  the  gradual  crystallization  of  the  individual  con- 
stituents, they  accumulate  in  the  concentrated  mother-liquor. 

116 


CONT ACT-MET AMORPHISM  117 

When  this  also  begins  to  solidify,  the  gaseous  material  is  set  free 
and  produces  certain  stresses  which  may  lead  to  explosions  and 
the  rupture  of  the  overlying  rock-complex.  In  other  cases  the 
cover  is  too  thick  and  resistant,  or  the  country-rock  is  so  full  of 
pores  and  other  channels  that  the  accumulated  gases  are  forced 
into  them,  not  only  along  fissures  or  schistosity-planes,  but  in 
such  a  manner  that  they  thoroughly  saturate  the  entire  rock. 

The  mineralizers,  now  agents  of  cont act-met amorphism,  gener- 
ally separate  from  the  granitic  magma  only  in  the  very  latest 
stage  of  its  solidification,  that  is  during  the  crystallization  of  the 
quartz  or  of  the  eutectic  quartz-feldspar  mixture,  and  these 
agents,  still  very  hot,  saturate  the  rocks  already  heated  by  the 
igneous  intrusion.  While  the  dry  heat  of  even  the  greatest  in- 


FIG.  71. — Greenstone-schist   with  porphyritic    palimpsest  texture.     Harthau,  near 

Chemnitz. 

trusive  masses  can  only  penetrate  to  relatively  slight  distances 
into  the  country-rocks,  owing  to  their  low  conductivity,  the  hot 
gases  pervade  them  for  long  distances.  This  contact-metamor- 
phism  by  magmas  rich  in  mineralizers,  may  extend  for  many 
kilometers  into  the  country-rock,  especially  if  these  have  been 
greatly  fractured  by  orogenic  processes,  as  is  seen  in  the  schist 
zones  around  the  central  Alps.  On  the  other  hand,  in  the  neigh- 
borhood of  basic  igneous  rocks,  such  as  gabbros  or  peridotites, 
which  are  poor  in  mineralizers,  the  contact- me tamorphic  effect 
disappears  completely  within  50  to  100  meters. 

The  mineralizing  agents  which  passed  from  the  intrusive  mass  into  the  country- 
rocks  produced  in  them  a  mobility  of  molecules  which  usually  led  to  re-arrangements, 
but  although  the  neighboring  rocks  were  saturated  by  the  agents  of  contact-metamor- 
phism  and  became  soft,  they  did  not  become  as  mobile  as  igneous  magmas.  This 
viscosity  is  shown  by  innumerable  phenomena.  For  example,  in  many  cases  the 


118         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

earlier  schistosity  of  sediments  remains  visible,  in  part  megascopically,  in  part  micro- 
scopically. This  is  especially  well  shown  in  the  helizitic  texture  (Fig.  3,  PL  IV),  in 
which  many  inclusions  remain  parallel  to  the  former  bedding  but  cut  through  the 
later  contact-minerals.  Coarser  clastic  constituents  in  most  cases  are  attacked  only 
around  their  peripheries,  and  still  distinctly  show  their  original  forms.  Furthermore, 
the  finest  structure  of  resistant  fossils  may  be  clearly  recognizable  in  greatly  altered 
rocks.  The  textures  of  porphyries  and  diabases,  also,  remain  clearly  visible  in  con- 
tact-metamorphosed greenstones  which,  but  for  this,  would  be  indeterminable  (Fig. 
71).  Palimpsest  texture  indicates  that  the  constituents  had  but  slight  mobility  during 
the  crystallization  of  the  contact-rock.  In  the  so-called  sieve  texture  (Fig.  1,  PI.  IV), 
the  larger  crystals  were  formed  in  a  viscous  magma.  While  abundant  inclusions  are 
rare  in  the  chief  constituents  of  igneous  rocks,  contact-minerals  may  be  so  filled  with 
them  that  the  chemical  composition  can  be  determined  with  difficulty. 

Contact-metamorphism,  as  a  rule,  does  not  alter  the  chemical 
composition  of  rock,  even  though  it  is  completely  metamorphosed. 
Simple  molecular  rearrangement,  therefore,  is  the  important 


iiil 

[4       |i 

^t^-^t*^ 

FIG.  72. — Tourmaline  impregnation  of  a  contact-rock.     Bayumkol  Valley,  Tienschan. 

factor.  Chemical  alterations,  such  as  the  loss  of  carbon  dioxide 
by  the  formation  of  silicates  in  carbonate  rocks,  the  loss  or  addi- 
tion of  water,  and  the  introduction  of  fluorine,  chlorine,  boron, 
and  other  mineralizers,  are  of  less  importance.  One  of  the  most 
important  changes  in  granite  contact-rocks  is  the  universal  im- 
pregnation with  tourmaline.  This  mineral  may  be  megascopic 
in  the  inner  zones,  although  ordinarily  it  impregnates  the  rock  in 
small  microlites.  It  is  even  present  in  argillites  which  are  so 
little  altered  that  there  is  only  a  suggestion  of  alteration  of  its 
clay-like  material  to  mica,  and  in  which  the  development  of  clay- 
slate  needles  has  barely  commenced.  Fig.  72  is  a  photomicro- 
graph of  an  altered  schist  containing  innumerable,  small,  well- 
developed  tourmalines.  Tourmaline  is  especially  valuable  for  the 


CONTACT-MET AMORPHISM  119 

recognition  of  contact-metamorphism  on  account  of  the  ease  with 
which  it  is  determined.  On  the  other  hand,  scapolite,  which  also 
is  indicative  of  contact-metamorphism,  is  determinable  with  diffi- 
culty, and  is  therefore  much  more  frequently  overlooked.  It  is 
also  very  likely  to  be  altered. 

Feldspar,  especially  albite,  may  be  secondary,  as  in  certain 
schistes  feldspathises,  the  materials  having  been  brought  in  by  the 
mineralizers.  But  feldspathization  by  this  means  is  doubtless 
much  less  common  than  is  usually  thought,  since  numerous  argil- 
lites  and  similar  rocks  probably  originally  contained  the  con- 
stituents necessary  for  the  formation  of  feldspar  by  molecular 
re-arrangement.  Finally,  many  sulphide  ores,  hematite,  etc.,  are 
secondary  in  contact-rocks. 

The  influence  of  the  country-rocks  upon  soda-rich  intrusives, 
such  as  nephelite-syenite,  is  in  many  cases  very  marked.  For 
example,  the  lazurite  content  of  lapis  lazuli,  an  altered  granular 
limestone  at  a  nephelite-syenite  contact,  must  be  ascribed  to 
mineralizers  from  the  alkali-rich  magma.  Further,  many  com- 
ponents of  the  igneous  magma  are  found  in  the  contact-minerals, 
and  fragments  torn  loose  from  the  country-rocks  are  re-formed 
into  sodium-,  zirconium-  and  titanium-rich  silicate  aggregates. 

Gases  naturally  diffuse  much  more  slowly  in  compact  country-rocks  than  in  those 
that  are  more  or  less  porous,  yet  under  the  influence  of  high  pressure,  extremely  mobile, 
highly-heated  gases  are  able  to  penetrate  and  re-form  the  densest  rocks.  Slow  as 
this  process  is,  it  distributes  the  heat  and  prevents  local  fusion.  Such  rocks,  therefore, 
as  the  plutonites,  from  which  the  mineralizers  escaped  gradually  as  the  magma 
crystallized,  generally  do  not  melt  the  rocks  with  which  they  are  in  contact,  but  pro- 
duce in  them,  to  a  considerable  distance,  a  molecular  re-arrangement  and  a  general 
recrystallization.  Where  these  mineralizers  rapidly  escape,  however,  as  by  the  rising 
of  extrusive  magmas,  a  complete  or  partial  melting  (fritting)  of  the  country-rock  may 
occur,  but  this  is  confined  to  the  part  lying  immediately  adjacent  to  the  igneous  contact. 
All  alterations  of  this  character  which  preexisting  rocks  undergo  at  the  contact  with 
igneous  rocks  are  embraced  under  the  term  contact-metamorphism. 

Contact-metamorphism  Produced  by  Plutonic  Rocks. — The 
contact-metamorphism  produced  primarily 'by  mineralizers  ema- 
nating from  deep-seated  rocks,  varies  greatly  in  intensity  and 
extent,  but  all  processes  of  contact-metamorphism  proceed  in  one 
direction,  namely,  to  produce,  for  the  given  chemical  and  physical 
conditions,  the  greatest  possible  stability  with  the  least  alteration  in 
the  chemical  constitution  of  the  rock. 

It  is  not  probable  that  all  granitic  magmas,  at  the  time  of  their  final  consolidation, 
were  equally  saturated  by  gases,  but  in  general  the  granitic  magmas  were  richer  in 


120         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

them  than  were  those  which  crystallized  as  gabbros  or  basalts.  Only  the  degree  of 
alteration  produced  in  the  country-rocks  was  influenced  by  these  relationships,  the 
kind  of  alteration  was  the  same  whether  the  rocks  were  basic  or  acidic.  Enormous 
intrusive  masses  of  granite.or  similar  rocks  naturally  gave  off  great  quantities  of  miner- 
alizers  during  their  slow  solidification,  and  these  eventually  saturated  and  heated  the 
country-rocks  to  great  distances.  Smaller  masses  gave  off  less  gas,  and  the  relatively 
rapid  cooling  soon  ended  the  anamorphism. 

With  the  exception  of  water,  which  is  always  predominant,  the 
effect  of  the  mineralizers  from  different  magmas  differed  greatly. 
Traces  of  such  agents  still  remain  in  the  contact  formations  of 
many  rocks,  especially  in  those  of  the  granite  family.  Thus  the 
presence  of  boron-  and  fluorine-bearing  tourmaline,  topaz,  fluorite 
and  other  fluorides,  chlorine-bearing  scapolite,  etc.,  suggest  the 
composition  of  the  agents  set  free  by  the  crystallization  of  the 
magma.  At  contacts  of  basic  rocks,  such  occurrences  are  excep- 
tional, scapolite  being  the  mineral  most  commonly  formed. 

The  extent  and  intensity  of  the  alterations  produced  by  con- 
tact-metamorphism  do  not  depend  solely  upon  the  amount  and 
nature  of  the  metamorphosing  agents  but  perhaps  even  to  a  greater 
degree  upon  the  permeability  of  the  country-rocks  and  the  ease 
with  which  they  can  be  recrystallized.  As  noted  above,  the 
metamorphic  agents,  being  under  high  pressure,  penetrate  all 
rocks,  no  matter  how  intimately  the  constituents  are  intergrown, 
yet  the  distance  to  which  the  metamorphism  extends  depends 
largely  upon  the  degree  of  consolidation  or  cementation. 

Rocks,  such  as  shales,  argillaceous  sandstones,  and  volcanic  tuffs,  whose  coherence 
is  relatively  slight,  and  rocks  which  were  originally  compact  but  were  subsequently 
greatly  fractured  by  tectonic  processes,  are  readily  saturated  to  great  distances.  Very 
compact  formations,  on  the  other  hand,  such  as  fine-grained  igneous  rocks  or  siliceous 
sandstones,  are  much  less  permeable. 

The  bedding-planes  of  rocks  offer  easy  passages  for  the  mineralizers.  Thin-bedded 
formations,  therefore,  are  in  many  cases  altered  to  great  distances,  the  greater  where 
the  beds  had  become  loose  or  porous  by  intense  folding.  Thin-bedded  rocks,  invaded 
by  igneous  stocks,  are  usually  much  shaken  and  faulted  by  the  magma  pressing  for- 
ward, and  these  effects  only  gradually  disappear  far  from  the  igneous  body.  As  a  result 
of  the  intrusion,  the  bedding  planes  were  opened  for  the  entrance  of  the  gases,  and 
saturation  took  place  to  surprising  distances,  especially  if  the  intrusive  material  was 
forced  between  the  beds  in  the  form  of  apophyses. 

Contact-metamorphism  is  not  so  great  in  thin-bedded  rocks  where  the  schistosity 
runs  parallel  to  the  contact,  for  example  in  the  roof  of  a  laccolith  (cf.  Fig.  5).  The 
beds,  raised  by  the  magma,  usually  show  many  fissures  across  the  stratification,  and 
into  these,  parts  of  the  magma  are  forced.  The  movement  of  the  mineralizers,  how- 
ever, is  generally  along  the  bedding-planes,  for  there  are  but  few  passages  at  right 
angles  to  this  direction,  consequently  the  intensity  as  well  as  the  extent  of  the 
contact-metamorphism  is  here  very  much  less. 


CONTACT-METAMORPHISM 


121 


On  the  other  hand,  very  extensive  alterations  occur  in  the  above  case  if  the  roof 
scales  off  into  the  melt,  or  where  a  schist  is  torn  apart  by  the  intrusive  mass  and  is 
injected  with  molten  matter  (cf.  Fig.  36).  The  injected  material,  being  usually  the 
very  mobile,  mineralizer-rich  mother-liquor,  passes  to  great  distances,  and  makes 
the  contact-metamorphic  alterations  intense  and  extensive. 

That  the  aplitic  apophyses  of  granite  are  igneous  may  be  clearly  seen  by  the 


FIG.  73. — Hornfels  showing  increased  metamorphism  at  the  c< 
Riesenberg,  near  Ossegg,  Erzgebirge. 


mtact  with  aplite  dikes. 


additional  alteration  of  the  contact-metamorphosed  rock  directly  at  the  margin  of  the 
dike.  This  is  shown,  in  the  andalusite-hornfels  of  Fig.  73,  by  a  darker  and  much  more 
distinctly  crystalline  zone  exactly  parallel  to  the  aplitic  apophysis. 

Finally,  the  end-members  of  aplitic  injections,  namely,  the  granular  quartz  dikes 
which  occur  in  the  zone  farthest  from  the  main  intrusion,  are  true  igneous  formations, 
as  is  shown  by  Fig.  74,  which  represents  a  knotenschiefer  in  which  the  cordierite 


FIG.  74.- 


-Knotenschiefer  showing  increased  metamorphism  at  the  contact  with  an 
igneous  quartz  dike.     Plauen,  Vogtland. 


nodules  have  undergone  a  considerable  enlargement  at  the  contact  with  the  injected 
pure  quartz. 

The  susceptibility  of  a  rock  to  alteration  by  contact-metamorphism  is  likewise 
an  important  factor.  Rocks,  such  as  certain  sandstones,  which  are  readily  permeable 
by  the  mineralizers,  are  capable  of  very  slight  rebuilding;  others  which  are  well  con- 
solidated, such  as  certain  basic  igneous  rocks  or  innumerable  limestones,  are  readily 
altered. 

Contact-metamorphism  is  hardly  noticeable  in  rocks  which  are  practically  in  a 


122         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

condition  of  stable  equilibrium  for  the  given  conditions ;  nor  are  rocks  whose  conditions 
of  formation  were  nearly  the  same  as  those  of  contact-metamorphism  much  influenced 
by  it.  For  example,  no  contact  alteration  appears  where  an  older  granite  is  intruded 
by  a  younger;  but  where  a  basic  igneous  rock  is  cut  by  a  granite,  distinct  alterations 
appear,  since  the  chemical  and  physical  conditions  under  which  the  two  were  formed 
were  different. 

Sedimentary  rocks  are  generally  much  altered  at  the  contact,  since  the  constituent 
minerals,  brought  together  at  random  by  the  sedimentation,  react  upon  each  other  to 
produce  various  minerals.  The  finer  the  individual  components  of  the  sediments,  the 
more  complete  will  be  the  mixing  of  the  individual  parts;  consequently  the  recrystal- 
lization,  which  depends  upon  their  reciprocal  reactions,  also  will  be  more  marked. 
Large  fragments  remain  almost  entirely  unaltered  so  that  the  clastic  character  of 
coarse  sandstones  and  conglomerates  is  generally  distinctly  preserved  even  after 
intense  contact-metamorphism.  Rocks  predominantly  of  quartz  are  least  altered, 
especially  when  the  individual  grains  are  of  considerable  size,  as  in  sandstones. 
Here  and  there  mineralizers  pass  through  a  whole  complex  of  sandstones,  poor  in 
cement  and  therefore  porous,  without  producing  any  externally  visible  alterations. 
It  is  true  that  even  in  these  apparently  unaltered  formations,  the  beginning  of  recrys- 
tallization  may  be  seen  under  the  microscope,  for  the  previously  rounded  quartz- 
grains  are  now  interlocked,  and  the  clay-like  interstitial  material  is  altered  to  a  mica- 
ceous mineral.  The  clastic  texture  in  these  rocks  appears  much  less  distinct  under  the 
microscope  than  it  does  megascopically.  Rocks  of  different  character,  however,  lying 
beyond  the  limits  of  the  sandstones,  may  be  greatly  altered  by  the  metamorphic  agents 
which  passed  through  the  latter  without  affecting  them. 

In  certain  exceptional  cases,  the  metamorphism  of  the  sediments  has  an  anomalous 
character.  For  example,  the  calcareous  country-rocks  at  the  contact  with  the 
monzonite  of  the  LeSelle  Pass  in  the  Monzoni  valley  are  normally  altered  to  silicate- 
rich,  granular  limestones  for  hundreds  of  meters.  Locally,  however,  in  the  place  of 
these  extensive  alterations,  there  occur  narrow  bands  of  siliceous  hornf  els  of  such  den- 
sity that  they  are  irresolvable  even  under  the  microscope.  In  other  regions,  along 
limestone  contacts,  instead  of  the  usual  recrystallization  products,  there  appear  very 
siliceous  dolomites  which  are  ordinarily  only  a  few  meters  wide  and  beyond  which  the 
sediments  are  practically  unaltered.  That  these  changes  were  not  due  to  erogenic 
movements  is  shown  by  apophyses  of  the  intrusive  rock  which  penetrate  the  dense 
hornf  els  and  dolomites. 

Inclusions  in  plutonic  rocks  are  usually  more  extensively  metamorphosed  than  is 
the  country-rock.  Besides  the  resorption  of  basic  inclusions  by  siliceous  magmas,  and 
acid  inclusions  by  those  that  are  silica-poor,  both  producing  very  unusual  rock-modi- 
fications, there  is  also  found  an  extensive  injection  and  saturation  of  the  inclusions  by 
the  magma.  Here  belong,  for  example,  the  so-called  gneiss  inclusions  in  granites, 
which  are  often  cited  as  proof  that  gneiss  occurs  at  great  depths.  They  are,  however, 
simply  metamorphosed  argillites  which  have  been  completely  saturated  with  igneous 
material. 

In  later  periods  of  volcanic  activity  the  contact-minerals  themselves  may  be  re- 
placed and  altered,  causing  the  contact-rocks  to  be  filled  with  pseudomorphs  and 
secondary  minerals,  such  as  serpentine,  talc,  kaolin,  brucite,  chlorite,  sericite,  etc. 

In  regard  to  names  for  contact-roeks,  those  here  used  are  more  or  less  accepted 
everywhere.  No  classification  that  is  generally  acceptible  has  been  proposed,  and 
the  naming  of  each  combination  of  secondary  minerals  would  produce  too  complicated 
a  nomenclature.  But  even  the  terms  in  use  are  not  especially  satisfactory.  For 
example,  the  word  hornfels  generally  carries  with  it  the  conception  of  compactness, 
yet  all  hornfels  is  not  compact. 


CONTACT-METAMORPHISM 


123 


Contact-metamorphism  of  Argillites. — Most  argillites  consist 
of  uniform  mixtures  of  various  very  finely-divided  constituents, 
and  therefore,  when  metamorphosed,  are  likely  to  contain  many 
kinds  of  secondary  minerals.  Since  argillites  are  very  permeable 
on  account  of  their  usual  complete  schistosity,  the  phenomena  of 
contaet-metamorphism  are  most  beautifully  and  typically  shown 
where  these  rocks  are  cut  by  igneous  stocks.  The  development 
of  altered  zones  at  the  contact  between  granite  and  argillites  is  so 
distinctive  under  normal  conditions  that  contact-metamorphism 
was  recognized  here  long  ago. 

Although  the  relationships  may  not  be  as  simple  as  was  formerly 
thought,  and  numerous  modifications  in  the  physical  conditions 


Outer  contact  zone  Inner  contact  zone  Granite 

FIG.  75. — The  Oberlauterbach  granite  stock  with  its  contact-zones.    Vogtland. 

may  make  the  meaning  of  the  formative  processes  difficult  to 
understand,  yet  normal  contact-metamorphism  of  argillites,  as 
the  prototype  of  all  contact-metamorphism,  must  be  described  in 
detail. 

Naturally  the  sedimentary  rocks  lying  nearest  the  igneous 
contact  (Fig.  75)  are  most  altered,  and  have  undergone,  in  part, 
a  complete  internal  re-arrangement.  On  this  account  the  schistose 
structure  is  completely  destroyed,  and  the  original  argillites  are 
now  hard,  dark,  hackly-fracturing  hornfels,  and  form  the  so-called 
contact-breccias  (Fig.  76).  Under  the  microscope,  the  rock  is 
completely  crystalline  and  there  is  only  a  suggestion  of  the  original 
schistosity  in  the  helizitic  texture.  Isolated  fragments  of  argillites, 


124         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

torn  loose  at  the  contact,  are  especially  greatly  altered,  and  are 
penetrated  by  veins  of  the  igneous  rock. 

Farther  from  the  contact,  the  haphazard  texture  gradually 
gives  place  to  one  that  is  distinctly  schistose  though  still  holo- 
crystalline.  Certain  minerals,  also,  develop  as  phenocrysts, 
usually  with  very  poor  boundaries,  and  the  rock  takes  on  a  por- 
phyritic  appearance.  Such  rocks  are  called  Knotenschiefer.  Still 
farther  from  the  contact  the  crystalline  appearance,  megascopically 
as  well  as  microscopically,  gradually  decreases,  and  through  all 
possible  transitions  normal  argillites  appear  in  the  outer  contact- 
zone.  However  unaltered  the  latter  rocks  may  appear  to  the 


FIG.  76. — Contact-breccia.     Geyer,  Erzgebirge. 

unaided  eye,  under  the  microscope  they  may  still  show,  for  a 
considerable  distance,  the  gradually  decreasing  effect  of  the 
contact-metamorphism. 

As  already  mentioned,  rocks  of  different  compositions  are  susceptible  to  metamor- 
phism  by  the  agents  of  contact-metamorphism  to  very  different  degree.  Thus  while 
the  different  members  of  the  great  Paleozoic  slate  series  appear  alike  externally,  in 
many  cases  there  is  actually  a  great  variation  in  composition  from  bed  to  bed.  Just 
as  kaolin  beds  may  be  interbedded  between  strata  which  were  originally  normal 
weathered  material,  for  example  in  the  Carboniferous  of  Ruhr  and  the  Rheinpfalz,  so 
also  may  there  be  extensive  changes  in  the  character  of  the  altered  rocks  in  one  and 
the  same  zone.  Hornfels  has  been  considered  to  be  the  exclusive  rock  of  the  inner 
contact-zone,  probably  primarily  because  contact  action  was  first  studied  in  heavily- 
timbered  mountain  regions  of  moderate  elevation.  Here  the  compact  hornfels  stands 
out  prominently  after  the  more  schistose  material  lying  between  has  weathered  out, 
and  the  residual  boulders  usually  consist  of  the  same  resistant  rock. 

Where  deep  cuts  have  been  made  in  such  contact-zones,  however,  the  different 
stages  of  alteration  of  the  alternating  beds  are  found  directly  at  the  granite  contact. 
This  line,  however,  is  not  simple,  but  complicated  by  innumerable  apophyses  and  by 


CONT ACT-MET AMORPHISM 


125 


the  effects  of  numerous  replacement  processes.     Fig.  77  shows  a  contact  between 
granite  and  various  schists  and  argillites. 

The  minerals  of  the  dense  hornfels  (Fr.  corneennes)  are  very 
numerous.     Quartz  is  probably  never  absent  from  the  rock,  and 


Granite    Hornfels    Mica-    Chiasto-    Knoten-    Slate 
with  ap-  schist   lite-schist   schiefer 

lite  dikes 

FIG.  77. — Contact  of  granite  with  various  schists. 

it  may  be  of  considerable  importance.  Next  in  importance  are 
the  aluminium  silicates  andalusite  and  sillimanite,  then  cordierite. 
Further,  almandite  and  staurolite,  both  usually  megascopically 


FIG.  78. — Knotenschiefer.     Tirpersdorf,  near  Olsnitz,  Vogtland. 

visible,  micas  and  chlorite,  spinel,  feldspars,  especially  albite  and 
more  rarely  orthoclase  and  microcline,  and  finally  prehnite,  law- 
sonite,  the  epidotes,  and  corundum  occur.  The  ores  are  ordinarily 
present,  ilmenite  as  an  original  constituent,  and  hematite,  pyrite, 


126         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

and  magnetite  secondary.  The  organic  material  originally  in  the 
schist  is  altered  to  finely-distributed  graphite,  which  may  literally 
fill  the  other  contact-minerals  as  inclusions,  and,  with  the  micas, 
mark  the  helizitic  texture  of  the  rocks.  Tourmaline  is  probably 
never  wanting  microscopically,  while  scapolite  may  be  seen 
megascopically  in  many  light-colored  spots  rich  in  inclusions. 

True  hornfels  entirely  disappears  at  a  distance  of  rarely  more  than  a  few  hundred 
meters  from  the  contact,  and  the  country-rock  becomes  more  and  more  schistose, 


FIG.  79.  —  Garbenschiefer.     Zemmgrund,  Zillertal. 


nat.  size.) 


ZsrJiuckau 


FIG.  80.— Profile  through  Fig.  75. 


forming  Hornschiefer,  and  this,  in  turn,  by  the  increasing  predominance  of  the  micas, 
takes  on  an  appearance  similar  to  mica-schist.  Darker  spots,  richer  in  graphitic 
material  and  usually  representing  poorly-developed  and  inclusion-rich  crystals  of  the 
silicates,  occur  indistinctly  upon  the  schistosity-planes  of  Fleckschiefer.  In  Knoten- 
glimmerschiefer  and  Knotenschiefer  the  spots  are  more  distinct.  The  last  two  are  also 
called  Fruchtschiefer  (Fig.  78),  probably  on  account  of  the  fruit-like  appearance  of 
the  knots.  The  minerals  of  these  knots  are  rarely  determinable  megascopically, 

although  in  chiastolite-schist  the  peculiar  dis- 
tribution  of  the  inclusions  may  indicate  anda- 
lusite.  Even  under  the  microscope  they  can- 
not always  be  determined;  they  usually  con- 
sist of  andalusite  and  cordierite.  Similar  to 
Knotenschiefer  are  Garbenschiefer  (Ger.  Garben,  sheaf,  Fig.  79)  which  are  characterized 
by  sheaf-like  aggregates  of  hornblende  upon  cleavage  surfaces. 

The  knots  and  the  luster  of  the  schistosity  surfaces  do  not  die  out,  in  many  cases, 
until  a  distance  of  several  kilometers  has  been  reached,  and  the  rocks  pass  first  in 
external  habit,  then  internally,  into  normal,  unaltered  argillites.  Not  until  this 
original  condition  appears  do  the  small  tourmalines,  which  are  characteristic  of  con- 
tact-metamorphosed rocks  to  their  farthest  outliers,  disappear. 

The  limits  of  the  different  contact-zones  are  rather  irregular.  They  do  not  form 
concentric  shells  around  the  igneous  body  but  are  developed  to  a  greater  or  less  extent 
in  different  places.  Certain  members  may  be  wholly  wanting  on  one  side,  while  on 
another  they  may  be  most  extensively  developed.  The  contact-zone  may  appear 


CONT ACT-MET AMORPHISM  127 

much  wider  at  the  surface  than  it  actually  is,  as  is  shown  by  Fig.  80,  which  is  a  cross- 
section  through  Fig.  75  along  the  line  indicated.  On  account  of  the  gentle  slope  of 
the  contact  of  the  granite  with  the  schist  at  the  left  of  the  diagram,  the  igneous  mass  is 
actually  much  nearer  the  surface  than  it  appears  to  be  from  the  outcrops. 

Contact-metamorphism  of  Carbonate -rocks. -^-Finely  lami- 
nated, schistose  rocks  and  those  which  were  much  fractured 
in  the  contact-zone,  permit  the  mineralizers  to  penetrate  to  great 
distances.  The  more  thickly-bedded  and  consolidated  carbonate- 
rocks  are  very  much  less  permeable,  yet  they  show  themselves 
especially  sensitive  to  metamorphism,  and  recrystallized  fragments 
of  these  rocks  may  be  found  embedded  in  very  slightly  altered 
schists  of  the  first  group. 

This  difference  appears  very  distinctly  in  the  inner  contact- 
zone.  Here  the  carbonate-rocks  have  taken  on  a  coarse-granular 
texture  in  contrast  to  the  usual  dense  hornfels.  Recrystallized 
limestones  and  dolomites  appear  quite  different  under  the  same 
conditions,  the  latter  being  usually  much  finer  grained  than  the 
former,  which,  when  pure,  may  consist  of  individuals  an  inch  in 
length  directly  at  the  contact.  Such  coarse-grained  dolomites 
are  rare,  however. 

The  organic  inclusions  in  the  original  rock  have  an  important 
influence  upon  the  size  of  grain  of  the  metamorphosed  carbonate- 
rocks.  They  alter  to  finely  divided  graphite,  and  when  this  is 
uniformly  distributed  through  the  altered  rock,  a  decided  diminu- 
tion is  produced  in  the  size  of  grain.  Thus  the  gray  to  black 
bands  in  carbonate-rocks  are  always  much  finer  grained  than  the 
white. 

Normal  contact-metamorphism  by  plutonic  rocks  produces 
no  essential  alteration  in  the  chemical  composition  of  carbonate- 
rocks.  Although  small  amounts  of  tourmaline,  fluorite,  or  scapo- 
lite,  or  even  of  secondary  quartz  or  albite,  occur  in  limestones  thus 
altered,  on  the  whole  the  addition  of  foreign  constituents  is  just 
as  rare  here  as  it  is  in  normal  contact-metamorphism  of  other 
rocks.  Pure  limestones,  therefore,  are  usually  altered  to  pure 
white  marbles.  In  argillites  the  different  substances  which  lie  in 
contact  with  each  other  usually  react  under  the  influence  of 
mineralizers  to  produce  new  minerals.  Such  contact-minerals 
do  not  appear  in  pure,  dense  limestones,  for  these  rocks,  before 
metamorphism,  consisted  of  a  fine,  crystalline  aggregate  of  calcite 
which  originated  by  diagenesis,  and  their  alteration  is  shown 


128         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

primarily  by  an  enlargement  of  grain  which  transforms  their 
megascopically  dense  structure  into  the  distinctly  crystalline 
texture  of  marble. 

Ordinarily,  however,  compact  limestones  are  not  pure,  but 
contain  a  greater  or  lesser  amount  of  clastic  aluminium  silicates 
which,  under  increased  temperature  and  with  the  assistance  of 
mineralizers,  react  in  various  ways  with  the  calcium  and  mag- 
nesium carbonates.  Normally,  the  carbon  dioxide  is  driven  off, 
and  the  original  impurities  may  form  large  crystals  of  calcium- 
aluminium  silicates  such  as  garnets,  vesuvianite,  epidote,  anorthite, 
and  mica — especially  phlogopite.  They  also  form  calcium-mag- 
nesium silicates  such  as  the  pyriboles,  magnesium  silicates  such  as 
forsterite  and  minerals  of  the  humite  group,  and  rarely  the  pure 
calcium  silicate  wollastonite.  Entirely  absent,  however,  in  these 


FIG.  81. — Marble  lens  in  gneiss,  formed  by  the  squeezing-together  of  a  thin  bed. 

Steinhag,  near  Passau. 

recrystallized  limestones  or  calciphyres,  are  the  typical  minerals 
of  the  argillaceous  hornfels,  above  all  aluminium  silicates  such  as 
almandite  and  staurolite.  Furthermore,  cordierite  is  wanting, 
but  spinels  or  even  corundum  may  occur. 

The  slight  mobility  of  the  substance  of  the  rock  during  meta- 
morphism  is  also  clearly  shown  here.  The  original  schistosity  is 
distinctly  preserved  in  many  cases,  especially  in  folded  and 
schistose  mica-cipolins,  and  the  fossil  remnants  of  echinoderms, 
corals,  belemnites,  etc.,  are  here  and  there  easily  recognizable, 
and  show  clearly  that  these  marbles  are  not  primordial.  The 
larger  contact-minerals  of  the  calciphyres  are  as  rich  in  inclusions 
as  are  the  knots  in  knotenschiefer,  and  their  rounded  edges  and 
pitted  surfaces  give  them  the  appearance  of  having  been  partially 
fused.  While  the  mineralizer-saturated  limestones  were  but 
slightly  mobile,  on  the  whole  they  were  very  plastic,  as  is  shown  by 


CONTACT-METAMORPHISM 


129 


the  adaptability  of  their  forms  to  their  less  plastic  enclosures. 
These  properties  show  why  inclusions  of  silicate  rocks  in  many 
granular  limestones  are  broken  into  fragments,  yet  are  everywhere 
surrounded  by  aggregates  of  carbonates  which  show  no  traces  of 
cataclastic  structure,  or  why  thin  layers  of  limestone  within  silicate 
rocks  are  squeezed  together  into  thick  lenses,  like  the  common 
inclusions  of  so-called  " primeval"  limestone  in  schists  (Fig.  81). 

The  contact-metamorphism  of  limestones  is  not  nearly  so  well  understood  as  is 
the  normal  contact-metamorphism  of  argillites.  This  is  so,  primarily,  because  so 
many  anomalous  phenomena  occur  at  granite-limestone  contacts.  It  is  not  un« 
common  to  find  peculiar  formations  between  the  two  rocks,  such  as  the  silicified  horn- 
fels  of  Monzoni,  or  the  compact  silica-rich  dolomite  described  on  page  122.  Likewise, 
the  constituents  of  the  magma  in  many 
cases  mix  with  those  of  the  limestone,  and 
lead  to  numerous,  peculiar,  silicate  aggre- 
gates. Finally,  at  the  contact,  there  may 
be  broad  zones  of  wollastonite  or  of  com- 
pact calcium-aluminium  or  calcium-mag- 
nesium silicates.  These  aggregates  may 
extend  far  into  the  marble,  in  part  in  the 
form  of  dikes,  in  part  as  very  irregular 
patches.  In  the  latter,  calcium  carbonate 
has  unquestionably  been  replaced  by  ma- 
terial derived  from  juvenile  solutions. 

The  granite  itself  may  show  endogenic 
modifications  at  the  contact  with  the 
marble,  and  there  may  be  within  it  a  rather 
broad  zone  of  garnet,  epidote,  vesuvianite, 
wollastonite,  pyroxene,  or  other  minerals 
which  usually  occur  in  isolated  crystals  in 
contact-metamorphosed  limestone.  Geolo- 
gists have  usually  regarded  these  minerals, 
which  are  doubtless  entirely  anomalous,  as 
the  normal  products  of  contact-metamor- 
phism of  limestone,  and  when  such  minerals  are  not  present  they  have  thought  the 
normal  contact-metamorphism  due  to  other  factors,  especially  dynamometamorphism. 
That  this  conception  is  erroneous,  and  that  these  minerals  are  not  necessarily  due  to 
contact-metamorphism  is  shown  by  the  occurrence  of  dikes  of  these  silicates  in  granular 
limestones.  For  example,  the  small  veins  of  dark  garnet  which  cut  the  wollastonite 
in  Fig.  82  clearly  show  the  secondary  character  of  the  entire  formation.  Small 
masses  of  such  silicate  aggregates  may  occur  in  the  limestone,  and  may  carry,  here 
and  there,  iron  sulphides  and  oxides.  In  places  these  deposits,  which  show  all  tran- 
sitional "phase's  to  marble,  may  be  of  such  extent  that  they  are  valuable  for  the  iron 
they  contain.  When  coarse  grained  they  are  called  skarn. 

Normal  contact-minerals  of  limestones  are  iron-poor  grossularite  and  clinozoisite, 
and  light-colored  diopside  and  tremolite,  while  the  silicates  of  the  dike-aggregates 
are  relatively  high  in  iron.  The  silicate  and  oxide  manganese  ores  of  many  granular 
limestones  have  skarn-mantles  rich  in  manganese-bearing  silicates,  while  the  skarn  of 
the  zinc-manganese  deposits  of  Franklin  Furnace,  New  Jersey,  contains  an  abundance 


I 


FIG.  82.— Garnet  dike  (dark)  with 
wollastonite  selvedge  in  marble.  Auer- 
bach  along  the  Bergstrasse. 


130         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

of  abnormal  zinc-manganese  silicates.  Since  these  minerals  are  abnormal  in  contact- 
metamorphosed  limestones,  they  could  only  have  been  brought  in  by  juvenile  waters. 
The  same  is  true  of  masses  of  siderite  and  magnesite  which  occur  similarly  in  contact- 
metamorphosed  carbonate-rocks. 

While  limestones  are  easily  altered  by  various  chemical  agents,  they  do  not  always 
change  so  far  as  to  be  unrecognizable.  Marbles  are  the  normal  contact-rocks  of  pure 
limestones,  and  calciphyres  and  cipolins  of  those  that  are  impure.  The  calciphyres 
contain  large  phenocrysts  or  aggregates  of  silicates,  spinel,  corundum,  tourmaline, 
fluorite,  scapolite,  and  blue  apatite;  the  cipolins  are  schistose  rocks  rich  in  mica 
and  chlorite.  With  the  exception  of  tremolite  and  wollastonite,  which  almost  ex- 
clusively occur  in  ray-like,* leaf y  aggregates,  the  contact-minerals  have  distinct  but 
incomplete  crystal  outlines,  and  may  be  of  considerable  size.  Ordinarily  they  are 
very  full  of  inclusions. 

The  more  impure  the  originally  dense  limestone,  the  greater  the  amount  of  newly 
formed  silicates.  In  the  alteration  product  of  calcareous  marl,  calcite  is  almost  or 
entirely  gone,  and  the  rock  now  consists  predominantly  of  xenomorphic-granular 
aggregates  of  calcium-aluminium  and  calcium-magnesium  silicates.  These  minerals 
usually  differ  from  those  of  the  skarns  in  their  small  content  of  the  heavy  metals, 
as  was  to  have  been  expected  from  the  composition  of  the  original  clastic  constituents 
of  the  dense  limestone,  derived  as  they  were  from  iron-poor  weathered  residues. 

Organic  substances  are  universally  distributed  through  sedimentary  limestones, 
and  they  may  be  altered  by  contact-metamorphism  to  graphite,  as  in  argillaceous 
hornfels.  This  mineral  occurs  in  granular  limestones  in  three  forms;  first,  as  fine, 
evenly  distributed  dust  through  the  rock,  producing  the  gray  to  black  color  of  marble; 
second,  in  small,  scaly  nodules,  and,  finally,  in  well-developed  crystal-plates  which  may 
be  over  a  centimeter  in  diameter.  The  first  variety  probably  came  in  most  cases  from 
original  organic  material  in  the  limestone;  the  second,  however,  which  is  ordinarily 
associated  with  dust-like  graphite  and  occurs  in  fissures  and  along  planes  of  movement 
of  the  rock,  is  probably  a  product  of  fumarolic  action.  The  third  variety,  which  is 
almost  always  associated  with  rich  graphite  deposits,  has  probably  the  same  mode  of 
origin  as  the  second.  All  of  the  original  organic  constituents  of  the  limestone,  however, 
are  not  altered  to  graphite  by  contact-metamorphism;  the  malodorous,  skatole-like 
constituent  of  stink-stone,  which  is  especially  resistant,  is  in  many  cases,  even  in  coarse- 
grained marbles,  preserved  entirely  unaltered  except  directly  at  the  contact.  Where 
the  evil  odor  disappears  delicate  blue,  rose-red,  or  lemon-yellow  tones,  which  fade 
readily  upon  exposure  to  light,  are  developed.  These  beautiful,  coarse-grained 
marbles  represent  the  stage  of  greatest  alteration.  Within  a  few  meters  they  are 
again  light  grey  in  color  and  malodorous  when  struck. 

Among  the  sedimentary  carbonate-rocks,  dolomitic  limestone  and  dolomite 
are  of  some  importance.  The  weathered  surfaces  of  many  limestones  which  have 
been  altered  by  contact-metamorphism  show  a  rough  surface  or  sandy  coating  of  nor- 
mal dolomite.  Microscopic  examination  of  the  fresh  rock  shows  clear  grains  of 
dolomite  among  cloudy  grains  of  calcite.  The  double  salt  dolomite  and  the  simple 
calcium  carbonate,  therefore,  crystallized  out  independently.  In  other  marble-like 
rocks,  such  as  predazzite  and  penkatite  from  Predazzo,  octahedrons  of  periclase  or 
pseudomorphs  of  brucite  after  periclase  occur  in  the  main  mass  of  granular  limestone. 
Here  the  more  easily  calcined  magnesium  carbonate  may  have  lost  its  carbon  dioxide 
by  heat. 

Pure,  granular,  dolomite  contact-rocks  are  widely  distributed.  In  texture  they 
are  usually  not  very  coarse,  and  they  are  always  finer  grained  than  the  adjacent  lime- 
stone. In  general,  they  are  rather  friable  on  account  of  the  tendency  of  the  dolomite 
to  crystallize,  and  many  of  them,  therefore,  are  decomposed  to  sandy,  so-called  "dolo- 


CONTACT-METAMORPHISM 


131 


mite-ash."  Many  kinds  of  contact-minerals,  such  as  tremolite,  the  magnesium  sili- 
cates forsterite  and  humite,  and  rarely  spinel  and  corundum,  are  found  in  granular 
dolomites. 

The  numerous  minerals  of  the  saccharoidal  dolomite  of  Binnental,  including  sul- 
phides and  sulphosalts  as  well  as  tourmaline,  are  probably  due  to  hot  juvenile  springs. 
It  cannot  be  determined  with  certainty  whether  these  and  other  contact-dolomites 
originated  from  dolomitic  sediments  or  from  limestones  to  which  magnesia  was  added, 
during  the  process  of  recrystallization,  by  the  action  of  the  agents  of  contact- 
metamorphism. 

Finally,  the  alteration  of  other  sedimentary  rocks  by  deep-seated  contact  action 
may  be  briefly  mentioned  here.  The  mineralizers  consist  predominantly  of  super- 
heated  water  which  simply  dissolves  the  different  salts.  Contact-minerals  are  found, 
here  and  there,  in  gypsum  and  anhydrite;  for  example,  phlogopite  occurs  with  tourma- 
line in  these  rocks  in  St.  Gotthard,  and  with  scapolite  in  the  Pyrenees. 

When  coal  is  altered  by  contact-metamorphism,  dense  graphite  is  formed,  but  the 
structure  of  the  coal  may  still  be  clearly  recognized  in  many  cases.  The  final  state  of 
normally  altered  coal  is  carbon-rich  anthracite,  which  crackles  or  even  explodes  upon 
heating  on  account  of  the  large  amount  of  gas  included. 

Contact-metamorphism  of  Basic  Igneous  Rocks.  —  The  magmas 
from  which  basic  igneous  rocks  are  derived  are  usually  poor  in 
mineralizers  .  These  rocks  there- 
fore almost  entirely  lack  the  hy- 
droxyl-bearing  minerals  of  the 
mica  and  amphibole  groups,  but 
in  their  place  occurs  pyroxene. 
Where  such  rocks  have  been 
metamorphosed  by  contact  with 
alkali-rich  rocks,  they  are,  in 
many  cases,  completely  recrys- 
tallized  by  the  mineralizers 
emanating  from  the  latter.  Anal- 
ogous alterations  OCCUr  in  the 

erpat  tuff  Hpnn^ifq  whiph  «r»pnm 

om~ 

pany    basic   extrusives,   and  it 

may  be  difficult  to  determine  the  original  character  of  rocks  which 

have  been  completely  altered  by  contact-metamorphism. 

Volcanic  tuffs  may  be  interbedded  with  various  sediments. 
When  typically  metamorphosed  basic  igneous  rocks,  such  as 
eclogites,  amphibolites,  or  greenstone-schists,  are  connected  by 
transition  members  with  interbedded  marbles  or  knoten-  or  mica- 
schists,  it  is  quite  certain  that  at  least  a  part  of  the  series  was 
originally  formed  by  tuffs.  Palimpsest  texture,  that  is,  one  in 
which  remnants  of  a  former  texture  are  recognizable,  may  suggest 
something  as  to  the  original  character  of  the  altered  rock.  Heli- 


FIG.  83.  —  Greenstone-schist  (metamor- 
Phosed  1  a  b  r  a  d  o  r  i  t  e  -porphyrite)  with 
phenocrysts  showing  good  boundaries. 


132         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

zitic  textures  indicate  originally  schistose  tuffs,  while  distinctly 
recognizable  ophitic  or  porphyritic  textures  (Fig.  83)  suggest 
massive  igneous  rocks. 

Of  all  rocks,  basic  igneous  rocks  and  their  tuffs  are  most  sensi- 
tive to  the  agencies  of  contact-metamorphism,  and  in  many  cases 
they  may  be  found  in  a  greatly  altered  condition  interbedded 
between  argillites  which  show  but  slight  traces  of  alteration. 
These  feldspar-bearing,  basic,  igneous  rocks,  such  as  gabbro, 
labradorite-porphyrite,  trap,  and  melaphyre,  are  characterized 
by  a  rather  low  percentage  of  silica  and  soda,  and  a  considerable 
amount  of  the  oxides  of  the  bivalent  metals  and  titanium.  The 
chief  mineral  constituents  are  a  soda-poor  aluminium-rich  plagio- 
clase,  a  soda-free  aluminium-poor  augite,  and  some  ilmenite. 

Where  most  completely  altered,  these  rocks  pass  into  eclogites,  which,  in  their 
purest  varieties,  consist  of  the  light-green,  soda-bearing  pyroxene,  omphacite,  and  an 
alumina-lime-iron  garnet,  near  almandite  in  composition.  Titanium  oxide  occurs  in 
the  form  of  microscopic  individuals  of  rutile.  Soda-bearing  hornblende,  which  marks 
the  transition  to  the  glaucophane  rocks,  is  of  primary  importance  among  the  in- 
numerable accessories.  The  complete  absence  of  feldspar  is  noteworthy,  the  soda 
content  having  passed  into  the  pyroxenes  and  amphiboles.  Every  trace  of  the 
original  texture  of  the  rocks  has  disappeared  in  the  intense  molecular  rearrangement 
of  the  constituents. 

In  most  cases,  however,  the  alteration  is  not  so  great,  even  directly  at  the  contact, 
and  amphibolites  are  formed.  In  these  rocks  the  saussuritized  plagioclase  and 
the  uralitized  pyroxene  are  usually  still  distinctly  separated,  one  from  the  other. 
Saussurite  is  composed  of  a  dense  aggregate  of  the  calcium-aluminium  silicates  cliho- 
zoisite  and  lime  garnet,  and  finely  divided  acid-plagioclase,  usually  albite.  Uralite 
is  ordinarily  common  green  hornblende.  In  many  cases  it  still  shows,  under  the 
microscope,  uniformly  distributed  brown  rods,  indicating  its  origin  from  diallage. 
Titanium  oxide  occurs  not  only  in  the  form  of  rutile  but  as  titanite  and  leucoxene, 
and  ilmenite  is  also  found.  All  possible  transitions  exist  between  eclogite  and  amphi- 
bolite,  the  most  common  intermediate  rock  being  garnet-amphibolite. 

Remnants  of  original  textures  in  amphibolites  may  be  distinctly  recognized  mega- 
scopically,  but  microscopically  they  are  less  clear.  Many  amphibolites  are  coarsely 
crystalline,  but  with  increasing  distance  from  the  granite  contact-zone  they  usually 
become  finer  grained  and  take  on  a  distinct  schistosity.  The  saussuritized  plagioclase 
phenocrysts  of  the  original  porphyrite  may  appear  quite  prominent  megascopically, 
otherwise  the  rocks  are  uniform,  compact,  green  schists,  whose  mineralogic  composi- 
tion is  only  recognizable  under  the  microscope.  When  they  have  a  considerable 
content  of  chlorite  they  are  called  chlorite-schists. 

Serpentinization  of  Feldspar -free  Rocks  by  Contact-metamor- 
phism.— Finally  to  be  considered  are  the  feldspar-free  igneous 
rocks,  among  which  peridotites  are  the  most  widely  distributed. 
The  great  majority  of  these  rocks  are  serpentinized.  They  gener- 
ally occur  in  compact,  lens-like  masses  concordantly  interbedded 


CONT ACT-MET AMORPHISM 


133 


with  the  schists  that  form  the  different  contact-zones  of  the 
granite.  Some  gneissoid  granites  contain  small,  rounded  masses 
of  serpentine  with  invariable  rims  of  chlorite  scales.  Within 
this  shell,  which  allows  them  to  drop  out  of  the  granite  quite 
readily,  there  is  usually  a  zone  of  radial  needles  of  fibrous  horn- 
blende. This  zone  stands  out  in  sharp  contrast  to  the  inclusion 
itself,  which  is  compact  and  is  composed  of  serpentine  with  talc 
and  chlorite. 

If  these  inclusions  are  compared  with  the  border-zones  of  certain 
central  Alpine  serpentines  whose  contacts  are  well  disclosed,  it 
will  be  found  that  they  are  generally  analogous  (Fig.  84).  The 
outer  zone  of  such  a  serpentine  mass  is  formed  of  coarse  scales  of 


FIG.  84. — Contact  of  serpentine,  Greiner,  Zillertal.  g,  Gneiss;  c,  coarse  chlorite; 
o,  talc  with  actinolite;  £,  talc  with  magnesite;  cm,  chlorite  rock  with  magnesite;  s,  ser- 
pentine. 

chlorite  surrounding  a  band  of  actinolite  standing  normal  to  the 
contact.  Within  this  is  a  transition-zone  of  talc  and  chlorite,  and 
finally  comes  the  compact  serpentine.  The  conclusion  appears 
justified  that  these  two  corresponding  modes  of  occurrence  have 
the  same  geologic  significance,  namely,  both  are  due  to  mineralizers 
from  granite  acting  upon  peridotite.  They  represent,  therefore, 
true  contact-zones,  the  peridotite  being  the  older  rock  and  the 
granite  the  younger. 

This  age  relationship  is  further  shown  by  the  fact  that  fresh  normal  peridotites 
never  carry  rich  ore  deposits;  the  mineral-veins  ordinarily  associated  with  serpentines 
being  entirely  wanting.  In  the  central  Alps  the  veins  cutting  the  serpentine  usually 
carry  minerals  which  were  entirely  foreign  to  the  original  peridotites,  and  which  bear 
no  chemical  relationship  to  them.  Normal  peridotites  are  free  from  titanium  and 


134         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

zirconium ;  in  the  central  Alpine  serpentines,  ilmenite,  titanite,  and  perof skite  are  not 
uncommon,  and  zircon  occurs  locally  in  large  crystals.  In  the  contact-formations  of 
all  basic  rocks,  and  especially  of  peridotites,  fluorine-  and  boron-bearing  minerals  are 
usually  entirely  wanting,  but  in  the  dikes  in  the  central  Alpine  serpentines  tourmaline 
is  not  rare,  and  in  many  cases  it  occurs  in  large  crystals.  These  phenomena  show  a 
consanguinity  between  the  dikes  in  the  serpentine  and  the  Alpine  Titan-formation, 
the  latter  belonging  to  the  post-volcanic  period  of  the  central  granite,  and  prove  that 
the  serpentine  is  older  than  the  granite. . 

Contrasting  with  the  innumerable  serpentine  masses  of  the  central  Alps  in  which 
remnants  of  original  olivine  are  only  rarely  preserved,  are  the  numerous  occurrences 
of  fresh  peridotites  in  the  Pyrenees  where  the  alteration  to  serpentine  is  exceptional. 
Here,  however,  these  feldspar-free  igneous  rocks  are  clearly  younger  than  the  granite. 

These  observations,  therefore,  seem  to  indicate  that  the  formation  of  serpentine 
depends  upon  emanations  from  a  younger  granite,  or  that  it  represents  the  normal 
contact-metamorphism  of  peridotite.  While  most  massive  serpentines  are  probably 
best  explained  on  this  hypothesis,  there  is  no  doubt  that  olivine  may  be  changed  to 
serpentine  by  the  action  of  water,  probably  exclusively  juvenile.  That  this  is  the 
case  is  clearly  shown  by  the  serpentinized  olivine  in  many  gabbros,  melaphyres,  and 
basalts;  but  it  may  be  definitely  asserted  that  this  serpentinization  was  not  produced 
by  atmospheric  agents. 

Piezo -contact-metamorphism. — If  the  phenomena  of  contact- 
metamorphism  be  observed  in  regions  where  orogenic  processes 
accompanied  the  solidification  of  the  igneous  rock,  innumerable 
modifications  will  be  noticed,  for  the  piezocrystallization  of  the 
igneous  rock  is  accompanied  by  piezo-contact-metamorphism 
of  the  country-rocks.  The  latter,  already  fractured  by  the 
intrusion,  were  generally  still  more  shattered  and  fragmented  by 
the  process  of  mountain-folding.  This  shattered  condition  and 
the  great  pressure  caused  the  mineralizers  to  enter  far  into  the 
rocks  and  made  the  contact-metamorphism  here  of  much  greater 
intensity  and  extent. 

The  effects  of  the  metamorphism  upon  the  different  constitu- 
ents varied  with  the  physical  conditions.  The  chief  constitu- 
ents of  the  recrystallized  rocks  show  the  effect  of  high  pressure 
by  their  greater  specific  gravities,  their  occasional  higher  hydroxyl 
content,  or  their  widespread  condition  of  schistosity.  In  common 
with  normal  contact-rocks  they  constantly  carry  tourmaline. 
Furthermore,  characteristic  textural  forms,  described  in  greater 
detail  in  Part  X,  are  especially  well  developed.  Garbenschiefer 
and  knotenschiefer  are  nowhere  more  characteristically  developed, 
nor  of  more  widespread  occurrence  than  in  the  schistose  border- 
zone  of  the  central  Alpine  granite.  The  changes  produced 
depended  primarily  upon  the  mineralogic  composition.  The  action 
of  the  law  of  volumes,  that  is  the  tendency  under  given  conditions 


CONT ACT-MET AMORPHISM  135 

for  a  body  to  occupy  the  smallest  possible  molecular  volume,  is 
clearly  shown. 

In  few  regions  are  granitic  intrusives  so  extensively  mingled 
with  the  surrounding  rocks  as  in  the  gneiss-mica-schist  zone  of  the 
central  Alps.  Here  true  mixed  or  hybrid  rocks,  called  migmatites 
(Gr.  /UTM<*,  a  mixture),  are  extensively  developed.  Apophyses  of 
the  granite  extend  far  from  the  intruded  mass,  and  aplites  and 
coarse-grained  pegmatites  are  encountered  many  kilometers  away. 
Dikes  and  beds  of  quartz,  in  many  cases  tourmaline-bearing,  are 
very  abundant  in  the  zone  of  the  less  crystalline  quartz-phyllite 
far  from  the  igneous  mass.  These  younger  intrusives  readily 
take  up  carbonates,  especially  ankerite  (Ca,Mg,Fe,Mn)CO3,  and 
the  most  distant  phyllite  zone  is  full  of  massive  quartz-ankerite 
dikes,  in  many  cases  coarse  in  texture. 

A  great  mass  of  evidence  having  shown  conclusively  that  the  crystalline  border- 
zone  of  the  central  Alps  is  not  of  Archean  age,  it  became  the  chief  region  for  demon- 
strating the  theory  of  dynamometamorphism.  The  volume-law  mentioned  above, 
and  the  wide  distribution  of  schistose  structures  in  the  border-zone  were  given  as 
special  proofs  of  dynamic  recrystallization.  There  is  good  reason  to  believe,  however, 
that  the  schistose  shell  is  of  piezo-contact-metamorphic  origin,  the  principal  evidence 
being  as  follows: 

The  innumerable  mechanically  formed  textures  (cf .  Part  X)  shown  almost  every- 
where by  the  granite,  and  the  schistose  character  of  the  border  in  many  parts,  led  to 
the  correct  conclusion  that  these  were  the  results  of  orogenic  forces,  but  these  features 
were  incorrectly  supposed  to  have  been  impressed  upon  the  rocks  long  geologic  periods 
after  they  had  been  completely  solidified.  Even  the  name  protogine  (Gr.  Trpwros, 
first,  yiyvonai,  to  be  born),  which  was  applied  to  these  rocks,  indicated  the  belief 
in  their  consolidation  as  a  part  of  the  original  crust  of  the  earth,  or  at  least  in  their 
great  age  as  compared  with  the  known  younger,  overlying,  crystalline  schist. 

On  petrographic  examination,  the  schistose  border  everywhere  shows  distinct  and 
indisputable  evidence  of  contact-metamorphism.  The  injection  schists  and  the 
true  hybrid-rocks  of  the  gneiss-mica-schist  zone  prove  not  only  that  these  are  older 
deposits,  but  that  the  granite  itself  is  a  later  intrusion  into  this  shell  of  former  sedi- 
ments and  basic  igneous  rocks.  The  same  age  relationship  between  the  two  rock- 
groups  is  shown  by  extensively  developed  pegmatites,  ramifying  aplites,  and  quartz- 
dikes.  At  the  contact  of  the  latter  with  the  schists,  an  increase  in  metamorphism  is, 
in  many  cases,  just  as  distinctly  recognizable  as  it  is  near  the  igneous  quartz-dikes  of 
normal  knotenschiefer.  Furthermore,  the  mosaic,  sieve,  helizitic,  and  other  textures 
characteristic  of  contact  action,  are  especially  well  developed  in  the  country-rocks 
of  the  central  Alpine  granite,  and  impregnations  of  tourmaline  extend  far  out  into 
rocks  otherwise  practically  unaltered. 

Here  as  elsewhere  in  contact-metamorphosed  regions,  the  metamorphism  is 
more  intense  the  nearer  the  rock  lies  to  the  granitic  intrusion.  Finally,  it  is  to  be 
especially  emphasized  that  mechanical  textures,  which  were  regarded  originally  as 
essential  to  dynamometamorphism,  are  entirely  wanting  in  the  altered  schists  of  great 
portions  of  the  central  Alpine  region.  Likewise  the  aplites,  which  represent  the  final 
extrusions  of  the  granitic  magma,  show  no  cataclastic  structure,  and  many  pegmatites 


136         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

contain  large,  open  druses  which  could  not  occur  if  these  rocks  were  contemporaneous 
with  the  orogenic  movements. 

Thus  there  is  strong  proof  that  the  crystalline  schists  of  the  central  zone  of  the 
Alps  were  formed  by  contact-metamorphism.  It  is  true  that  this  metamorphism 
must  have  taken  place  under  anomalous  conditions,  but  the  mineral  composition  of 
the  central  granite  itself  points  to  like  anomalous  relationships,  namely,  of  especially 
high  pressures  during  the  solidification  of  the  magma. 

Andalusite  and  cordierite,  the  commonest  minerals  of  normal  slate-hornfels,  are 
entirely  wanting  among  the  essential  constituents  of  the  Alpine  schist-zone.  They 
occur  here  only  as  constituents  of  pegmatites  whose  marked  drusy  character  definitely 
shows  that  pressure  had  ceased  when  they  solidified.  Instead  of  these  minerals,  piezo- 
crystallization  produced  micas,  especially  the  heavier  brittle  micas,  and  garnet,  zoisite, 
and  staurolite,  the  last  three  frequently  leading  to  the  formation  of  knotenschiefer. 
Disthene  in  sheaf-like  aggregates  on  schistosity-planes  also  occurs.  Where  mica 
predominates,  very  schistose  rocks  take  the  place  of  hornfels;  mica-schist,  therefore, 
is  the  usual  rock. 

The  modifications  produced  by  the  metamorphism  of  carbonate-rocks  are  entirely 
analogous.  Under  the  existing  high  pressures,  the  carbon  dioxide  of  the  calcite  was 
not  replaced  by  silica,  and  quartz,  in  rounded  crystals  similar  to  those  found  in 
quartz-porphyries,  is  here  a  widely  distributed  constituent.  Calcium-aluminium 
-silicates  are  rare.  Magnesium  silicates  are  only  developed  in  exceptional  cases,  but 
in  their  place  occur  albite  and  the  micas,  and  in  very  impure  rocks  abundant  epidote 
also.  Amphibole  is  almost  the  only  calcium-magnesium  silicate  present.  The  schis- 
tose structure  is  distinct  in  the  more  impure  varieties  of  this  group,  the  type-rock 
being  a  calcareous  mica-schist.  The  purer  varieties  of  calcareous  rocks,  under  similar 
conditions,  developed  a  compact  texture,  as  in  marble. 

Paragenesis  of  Contact -rocks. — Various  combinations  of  the 
chief  minerals  of  contact-rocks  have  produced  a  great  variety  of 
forms. 

1.  Characteristic    of    slate-hornfels    are:     Corundum,    spinel, 
andalusite,     sillimanite,     staurolite,     cordiertite,    mica,    chlorite, 
almandite,  hornblende,  zoisite-epidote,  rutile,  anatase,  ilmenite, 
graphite,   quartz,   and  various  feldspars.     Under  high  pressure, 
disthene,  brittle  mica,  etc.,  take  the  place  of  andalusite,  sillimanite, 
and  cordierite. 

2.  Characteristic   of    contact-limestones    and    calcium-silicate 
rocks  are:     Calcite,  dolomite,  periclase,  spinel,  grossularite,  vesu- 
vanite,    gehlenite,    zoisite-epidote,    anorthite,    diopside,    fassaite, 
phlogopite,  actinolite,  pargasite,  forsterite,  humite,  wollastonite, 
titanite,  and  graphite. 

3.  Characteristic    of    saussurite-    and    diabase-hornfels    are: 
Zoisite,    epidote,    vesuvianite,    lawsonite,   prehnite,   grossularite, 
albite,  fassaite,  glaucophane,  wulfenite,  chlorite,  common  garnet, 
disthene,  and  rutile. 

To  each  of  these  groups  may  be  added  some  of  the  minerals 
developed  by  the  action  of  mineralizers.  Among  these  are  tourma- 


CONTACT-METAMORPHISM  137 

line,  topaz,  fluorite,  scapolite,  apatite,  pyrrhotite,  pyrite,  mag- 
netite, etc.  Finally,  the  replacement  products  of  contact-minerals, 
which  will  be  spoken  of  later,  may  also  occur. 

In  conclusion,  a  few  of  the  laws  of  association  which  seem  to  hold  for  contact- 
metamorphism  may  be  given. 

1.  Andalusite,  sillimanite,  disthene,  staurolite,  cordierite,  and  the  brittle  micas 
are  wanting  in  calcite-bearing  contact-rocks. 

2.  Grossularite,  clinozoisite,  gehlenite,  and  vesuvianite  are  met  with  only  in  lime- 
rich  rocks. 

3.  Forsterite  and  the  humites  are  formed  in  granular  limestones  only  when  no 
alumina  remains,  otherwise  calcium-magnesium  silicates  and  spinel  are  developed. 

4.  Wollastonite  forms  only  in  rocks  which  are  very  rich  in  lime,  and  even  here  it  ? 
only  occurs  when  no  foreign  substances  except  silica  are  present. 

5.  Calcium-magnesium  silicates  occur  also  in  rocks  very  rich  in  aluminium. 

6.  Rutile  is  especially  abundant  in  rocks  which  are  rich  in  aluminium,  and  then  it 
occurs  in  association  with  the  minerals  mentioned  under  (1).     It  is  rarer  in  calcium- 
aluminium  silicate  rocks. 

Contact-metamorphism  by  Extrusive  Rocks. — The  character 
of  the  contact-metamorphism  by  extrusive  rocks  is  somewhat 
different.  Here  the  action  of  heat  is  the  most  apparent  effect, 
for  some  of  the  rock-constituents  are  melted  and  the  altered  rock 
appears  half  glassy.  Rocks  partially  melted  in  this  manner  are 
spoken  of  as  fritted,  and  there  are,  for  example,  fritted  sandstones 
in  which  the  cement  is  melted,  and  fritted  granite  with  its  mica 
and  a  part  of  its  feldspar  altered  to  glass.  Other  changes  pro- 
duced by  contact-heat  are  the  alteration  of  coal  to  coke,  and 
the  production  of  prismatic  parting  in  all  kinds  of  rocks. 

In  the  re-fused  portions  of  fritted  rocks,  microscopic  examina- 
tions show  that  minute  new  minerals  have  been  formed.  In  some 
cases  these  may  be  determinable  as  cordierite,  sillimanite,  or  spinel, 
but  usually  they  are  so  small  that  they  appear  only  as  crystallites. 
It  is  only  at  the  contact  of  extrusive  rocks  with  limestones  that  a 
re-formation,  analogous  to  that  at  the  contact  of  plutonic  rocks, 
is  ever  to  be  observed,  and  even  here  it  appears  only  in  very  narrow 
zones.  Granular  marbles  may  here  and  there  be  developed  in 
this  manner. 

Accompanying  the  partial  melting  there  also  occurs,  in  many  cases,  a  silicification 
of  the  rock,  as  in  porcelain-jasper  (porcellanite) .  The  occurrence  of  silicified  wood  at 
many  points  of  contact  with  extrusive  rocks  is  also  noteworthy,  and  adinole,  which  is 
found  at  many  diabase-contacts,  should  also  be  considered  here.  The  most  striking 
change  in  the  alteration  of  the  original  argillite  to  adinole  is  the  increase  of  as  much  as 
10  per  cent,  in  the  soda  content.  Chemically,  therefore,  there  is  here  a  certain 
analogy  with  albite-gneiss,  the  greater  part  of  whose  albite  was  brought  in,  sub- 
sequent to  deposition,  by  the  agents  of  contact-metamorphism.  Adinoles  as  well 


138         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

as  porcellanites  are  dense,  hornfels-like  rocks.  The  former  are  usually  mottled  and 
grey  in  color,  the  latter  violet  or  reddish  brown.  With  increasing  distance  from  the 
diabase-contact,  they  grade  through  desmosites  and  spilosites,  which  represent  less 
altered  and  more  schistose  formations,  into  normal  argillites.  Another  rock  which 
occurs  at  diabase-contacts  is  siliceous  schist  or  lydite,  a  black,  dense,  splintery  fractur- 
ing, greatly  silicified  rock  with  hardly  noticeable  schistose  structure.  All  of  these 
forms  of  alteration  have  a  similarity  to  the  anomalous  alterations  in  deep-seated  rocks 
noted  on  page  122. 

The  molten  magma,  forcing  its  way  to  the  surface,  may  bring 
up  many  solid  fragments.  These  may  be  hurled  out  in  the  form 
of  bombs  and  lapilli  and  appear  in  the  volcanic  tuffs,  or  they 
may  occur  as  inclusions  in  the  solidified  lava.  Some  of  the  inclu- 
sions and  ejectamenta  originated  during  the  differentiation  by 
slow  cooling  of  the  magma  itself  while  it  was  still  within  the  depths 
of  the  earth.  Such  primary  differentiation  products  are  espe- 
cially abundant  in  lamprophyres  and  basalts,  and  are  probably 
never  wanting  in  them.  They  also  occur  in  other  extrusive  rocks, 
generally  in  the  form  of  granular  masses,  such  as  those  of  sanidinite 
in  trachytes,  etc.  Other  fragments  are  portions  of  the  country- 
rock  which  were  torn  loose  in  the  depths  of  the  earth  by  the  magma 
during  the  earliest  stages  of  its  solidification.  Here  belong  gneiss 
inclusions  in  plutonic  rocks  and  certain  fragments  enclosed  in 
the  older  lavas  of  Vesuvius.  The  latter  were  thoroughly  satur- 
ated by  the  magma  and  its  mineralizers,  and  are  extremely  rich 
in  pneumatolytic  minerals.  The  composition  of  the  alkali-rich 
lava  in  which  they  are  enclosed  had  a  marked  influence  upon  their 
constituents,  so  that,  like  the  sanidinites,  they  ordinarily  contain 
alkali-rich  silicates. 

Rock-fragments,  on  the  other  hand,  torn  from  the  conduit-walls  by  the  volcanic 
magma  on  its  way  to  the  surface,  and  therefore  representing  inclusions  more  recent 
than  those  just  described,  show  especially  the  action  of  heat.  Among  such  rocks  are 
fritted  granites  and  sandstones,  in  many  cases  with  rod-like'  partings,  and  baked  clays. 
Many  such  inclusions  are  assimilated  and  resorbed,  and  now  appear  in  the  form  of 
glass  lumps  in  rocks  otherwise  distinctly  crystalline.  Lamprophyres  not  uncommonly 
contain  rounded  and  porous  orthoclase  crystals,  or  quartz-eyes.  The  former  are  the 
so-called  chagrined  feldspars,  and  seem  to  have  been  corroded  by  the  magma,  the  latter 
are  rounded  grains  with  glassy  rims  filled  with  radial  needles  of  augite  or  hornblende. 


VIII.  POST-VOLCANIC  PROCESSES 

LITERATURE 

E.  DE  BEAUMONT:  "Sur  les  emanations  volcaniques  et  metalliferes."     Bull.  Soc. 

Geol  France,  IV  (1847),  1249. 
R.  BECK:  " tJber  die  Beziehungen  zwischen  Erzgangen  und  Pegmatiten."     Zeitschr. 

f.  prakt.  Geol,  1906,  71. 
W.  C.  BROGGER:  "On  the  Formation  of  Pegmatite  Veins."     Canad.  Rec.  Sc.,  VI 

(1894),  33,  61. 
A.  DAUBREE:  "Observations  sur  le  metamorphisme  et  recherches  experimentales  sur 

quelques-uns  des  agents  qui  ont  pu  le  produire."     Ann.  d.  Mines.  Ser.  (5),  XII 

(1857),  294. 
R.  DELKESKAMP:  "Juvenile  und  vadose  Quellen."     Balneolog.  Zeitschr.,  XVI  (1905), 

No.  5. 
Idem:  "Vadose  und  juvenile  Kohlensaure."     Zeitschr.   prakt.  Geol.,   XIV    (1906), 

Hf.  2. 

F.  POSEPNY:  "Die   Genesis  der  Erzlagerstatten."     Berg-Huttenm.   Jahrb.,   XLIII, 

(1895),  1. 

FR.  SANDBERGER:  "  Untersuchungen  iiber  Erzgange."     Wiesbaden,  1882-1885. 
TH.  SCHEERER:  "tlber  die  chemischen  und  physischen  Veranderungen  kristallinischer 

Silikatgesteine  durch  Naturprozesse."     Ann.  Chem.  Pharm.,  CXXVI  (1863),  .1. 
M.  B.  SCHMIDT:  "Untersuchungen  iiber  die  Einwirkung  der  schwefligen  Saure  auf 

einige  Mineralien  und  Gesteine."   v  Tscherm.  min.  petr.  Mitteil.,  IV,  (1882),  1. 
A.  W.  STELZNER:  "Die  Lateralsekretionstheorie  und  ihre  Bedeutung  fur  das  Pfi- 

bramer  Ganggebiet."     Berg-Huttenm.  Jahrb.,  XXXVII  (1889). 
E.  SUESS:  "tvber  heisse  Quellen."     Verh.  Ges.  deutsch.  Natuf.  u.  Arzte.     Karlsbad, 

1902. 
C.  R.  VAN  HISE:  "Some  Principles  Controlling  the  Deposition  of  Ores."     Trans. 

Amer.  Inst.  Min.  Eng.,  XXX  (1900). 
J.  H.  L.  VOGT:  "Beitrage  zur  genetischen  Klassifikation  der  durch  magmatische  Dif- 

ferentiationsprozesse  und  der  durch  Pneumatolyse  entstandenen  Erzlagerstatten. 

II.   Pneumatolytische  bzw.  pneumatohydatogene  Produkte."     Zeitschr.   prakt. 

Geol,  1895. 

Post-volcanic  Phenomena. — Succeeding  the  true  volcanic 
action,  that  is,  the  extrusion  of  molten  material,  came  various 
chemical  changes,  some  of  them  very  intense,  which  may  be 
considered  together  under  the  name  of  post-volcanic  processes. 
The  first  changes  following  the  eruption  were  caused  primarily 
by  hot  gases  and  vapors,  and  are  therefore  called  pneumatolitic 
(Gr.TTpevMa,  vapor,  Aueiy,  to  set  free).  In  a  second  phase,  represent- 
ing a  stage  of  decreased  volcanic  energy,  the  changes  were  pro- 
duced by  superheated  solutions.  This  pneumatohydatogenic 
(Gr.  v8op,  water)  period  generally  passed  into  the  thermal  (Gr. 
0ep/uo5,  hot)  stage  of  hot  springs.  Geologically,  these  three  post- 
139 


140         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

volcanic  processes  represent  stages  of  decreasing  activity,  which 
have  passed  with  such  extreme  slowness  that  they  have  endured 
through  whole  geologic  periods.  The  principal  gases  and  vapors 
brought  up  by  the  so-called  fumaroles  are  compounds  of  sulphur, 
boric  acid,  hydrochloric  acid,  hydrofluoric  acid,  and  carbon 
dioxide.  Hot  springs  contain  the  same  agents,  usually  in  com- 
bination with  alkalies  and  alkaline  earths,  and  many  also  carry 
silicic  acid,  various  heavy  metals,  etc. 

When  post-volcanic  action  occurs,  it  is  intimately  connected 
in  time  with  contact-metamorphism,  but  its  effect  upon  the 
chemical  composition  of  the  rocks  is  usually  much  more  marked 
than  is  that  of  the  contact  action.  It  not  only  alters  the  meta- 
morphosed rocks  adjacent  to  the  volcanic  masses  which  it  accom- 
panies, but  in  many  cases  produces  a  great  change  in  the  igneous 
rocks  themselves.  Post-volcanic  phenomena  are  closely  related 
in  time  to  volcanic  action,  and  always  occur  near  the  igneous 
contact,  the  most  intense  action  being  nearest  the  contact.  The 
processes  by  which  these  agents  produce  changes  in  a  rock  are 
called  replacement  processes.  Contrasted  with  weathering,  which 
is  regional,  replacement  is  of  purely  local  significance. 

Post-volcanic  processes  may  form  new  minerals,  or  metamor- 
phose those  already  present,  or  destroy  them  entirely.  On  the  one 
hand  they  produce  most  of  the  new  minerals  occurring  in  mineral- 
or  ore- veins,  on  the  other  they  change  the  character  of  the  rock 
itself.  The  latter  change  may  be  a  simple  molecular  re-arrangement 
such  as  occurs  in  saussuritization,  and  therefore  analogous  to 
contact-metamorphism;  or  it  may  be  a  chemical  alteration  pro- 
duced by  the  leaching  of  some  of  the  constituents  as  in  kaoliniza- 
tion,  by  the  addition  of  material  as  in  tourmalinization,  or  by  an 
interchange  of  certain  components  as  in  the  formation  of  talc. 

Less  is  known  of  the  changes  on  the  border-line  between 
weathering  and  replacement  of  rock  constituents  than  of  any  other 
phase  of  chemical  geology.  Most  alterations  of  minerals  and  rocks 
have  been  ascribed  simply  to  the  action  of  atmospheric  agents, 
just  as  formerly  the  formation  of  ore-  and  mineral- veins  was 
ascribed  exclusively  to  the  leaching  of  the  country-rocks  by  vadose 
waters.  Even  the  most  typical  post-volcanic  formations,  such 
as  tin-veins  or  pegmatite,  were  considered  the  results  of  lateral 
secretion,  and  even  at  the  present  time  most  alterations  in  minerals 
are  ascribed,  without  further  consideration,  to  weathering. 


POST-VOLCANIC  PROCESSES  141 

A  cycle  of  movement  for  vadose  water  has  been  artificially  constructed  to  explain 
post-volcanic  processes,  but  it  is  based  upon  very  little  actual  observation.  The 
carbon  dioxide  laden  water,  falling  from  the  atmosphere,  is  supposed  to  be  able  to 
dissolve  and  destroy  many  minerals,  primarily  by  removing  their  calcium-  and  alkali- 
carbonates.  It  then  seeps  into  the  depths  through  the  capillaries  of  the  rocks,  its 
solvent  power  increasing  more  and  more  with  increasing  temperature.  The  circu- 
lating ground-water,  heated  or  even  in  part  greatly  superheated  in  this  manner, 
is  thus  supposed  to  saturate  the  earth's  crust  to  depths  where  the  critical  temperature 
of  water  is  reached,  that  is,  to  approximately  12  km.  These  heated  waters,  after 
withdrawing  various  substances  from  the  rocks,  come  together  in  open  fissures,  where 
they  are  again  forced  upwards  by  steam-pressure.  As  they  rise,  they  gradually  cool 
and  lose  their  solvent  power,  the  dissolved  ores  are  deposited,  and  the  water  itself 
perhaps  finally  reaches  the  surface  as  a  mineral-spring. 

A  strong  argument  against  this  hypothesis  is  the  fact  that  the  rocks  in  all  deep 
mines,  even  as  near  the  surface  as  several  hundred  meters,  are  dust-dry.  This  fact 
seems  to  show  conclusively  that  ground-water  is  no  longer  present  at  depths  giving 
an  increase  of  less  than  20°.  A  general  saturation  of  the  rocks  to  depths  of  many  kilo- 
meters, therefore,  is  excluded,  while  the  presence  of  open  fissures  so  far  down  is  highly 
improbable. 

Still  another  fact  shows  that  most  of  the  deposits  cannot  be  accounted  for  on  the 
ground-water  or  lateral  secretion  theories.  Although  extremely  small  amounts  of 
the  minerals  contained  in  the  tin-veins  are  undoubtedly  present  as  original  constituents 
in  the  surrounding  granites,  it  would  be  remarkable  if  the  vadose  waters  withdrew 
from  the  deep-seated  rocks  the  very  minerals,  such  as  cassiterite,  tourmaline,  etc., 
which  are  soluble  in  them  with  the  greatest  difficulty,  no  matter  what  the  temperature. 
The  problem  becomes  still  more  difficult  with  deposits  of  sulphide  ores,  for  the  asso- 
ciated igneous  rocks,  when  fresh,  never  contain  sulphur.  This  mineral  first  appears 
as  a  replacement  product  accompanying  ore  deposition.  Finally,  the  amount  of 
heavy  metals  accumulated  in  certain  ore  deposits  is  so  great,  as  compared  with  that 
distributed  through  the  country-rocks,  that  it  is  necessary  to  assume  the  leaching  of 
enormous  rock  masses,  such  as  are  not  always  at  hand,  to  explain  them.  Careful 
observation,  therefore,  leads  to  the  conclusion  that  most  ore  deposits  are  the  results 
of  juvenile  agents. 

The  atmospheric  agents,  working  from  above,  cover  the  rocks  with  a  mantle  of 
weathered  material.  This  is  usually  thin,  but  it  extends  to  greater  depths  where 
fractures  in  the  rock  permit  the  meteoric  water  to  enter,  or  locally  where  easily  soluble 
minerals,  such  as  the  sulphides,  are  the  source  of  active  agents,  as  in  the  gossan  of  ore 
deposits.  The  agents  of  replacement,  however,  act  from  below.  Their  products 
form  no  superficial  shell  and  do  not  necessarily  reach  the  surface.  Replacement 
phenomena,  therefore,  are  generally  of  equal  intensity  at  all  depths,  and  are  the  best 
criteria  of  post-volcanic  processes.  Compact  rocks  are  but  slightly  permeable  by 
the  agents  of  weathering,  the  penetration  of  atmospheric  waters  being  a  proof  of  poros- 
ity. The  agents  of  vulcanism,  on  the  other  hand,  are  much  more  able  to  attack 
the  rocks  on  account  of  the  increased  temperature  and  pressure  under  which  they  act, 
and  also  by  their  content  of  chemically  active  substances  which  may  be  present  in 
great  quantity.  Then,  too,  the  post-volcanic  agents  may  be  very  rich  in  these  chem- 
ically active  substances,  and  while  they  are  local  in  their  action  and  occur  only  in  close 
connection  with  other  volcanic  phenomena,  they  are  not  limited  to  the  superficial 
parts  of  the  earth's  crust. 

Attempts  have  been  made  to  determine  the  nature  of  the  re-forming  processes 
by  the  character  of  the  newly-formed  products.  Cornu  advanced  the  hypothesis 
that  crystalloids  result  from  post-volcanic  processes  while  colloids  are  produced 


142         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

by  atmospheric  weathering.  In  Chapter  V  it  was  pointed  out  that  the  most  impor- 
tant normal  weathering  products  of  the  feldspar  are  doubtless  colloids,  and  the  col- 
loidal nature  of  .limonite  and  numerous  other  materials  of  weathering  is  especially 
distinct  in  the  gossan  of  ore  deposits. 

Cornu's  hypothesis  fails  of  broad  application,  however,  as  may  be  easily  shown. 
Where  silica,  resulting  from  some  vadose  process,  separates  from  atmospheric  agents, 
it  always  takes  the  form  of  quartz,  as  does  also  the  originally  amorphous  silica  of  or- 
ganisms when  it  appears  in  fossils.  Further,  the  silica  deposited  in  rocks  by  atmos- 
pheric agents,  and  in  fissures  by  vadose  waters,  is  always  crystalline.  On  the  other 
hand,  silica  appears  in  the  form  of  opal  in  innumerable  post-volcanic  processes.  In 
this  form  it  is  always  associated  with  certain  igneous  rocks,  and  while  it  may  occur 
in  sediments,  its  origin,  in  every  case,  may  be  directly  traced  to  nearby  thermal 
action. 

Innumerable  other  amorphous  silicates  such  as  allophane  and  gymnite,  and  the 
hydrous  silicates  of  nickel,  garnierite  and  pimelite,  which  are  usually  amorphous, 
are  found  exclusively  in  areas  which  have  suffered  intense  post-volcanic  metamor- 
phism.  The  general  appearance  of  talc  suggests  that  it  was  originally  colloidal  but  later 
became  crystalline.  It  likewise  belongs  here,  as  do  also  meerschaum  and  innumerable 
other  magnesium,  magnesium-aluminium,  and  aluminium  silicates  which  are  amor- 
phous or  in  the  first  stages  of  recrystallization.  Finally,  chalcedony,  which  also 
appears  in  connection  with  volcanic  processes,  must  be  looked  upon  as  having  been 
originally  amorphous.  Amorphous  aluminium  phosphate,  such  as  turquoise,  and  the 
colloidal,  porcelain-like,  and  dense  magnesite  likewise  occur  only  under  conditions 
which  make  their  formation  by  juvenile  waters  probable. 

These  facts  show  that  it  is  impossible  to  separate  the  two  most  important  processes 
of  rock-destruction  on  the  basis  of  the  crystalloidal  or  colloidal  form  of  the  newly 
formed  minerals. 

Formation    of  Pegmatite. — Pegmatites  (Gr.  ^jua,  anything 
fastened   together,    on   account   of   the   frequent   occurrence   of 

graphic  granite)  are  intermediate 
between  true,  massive  rocks  de- 
rived from  normal,  mineralizer- 
saturated,  igneous  magmas,  and 
rocks  resulting  from  purely  pneu- 
matolytic  processes.  They  are 
satellites  of  normal  igneous  rocks, 
and  have  chemical  compositions 
similar  to  their  parent  magmas. 

They  are  usually  extremely  irreg- 
ular  in  form  (Fig>  85)j  and  are 

found  within  the  igneous  rock 
itself,  or  in  the  surrounding  contact-metamorphosed  zone,  in  the 
form  of  dikes,  schlieren,  or  independent  nodules.  Where  they 
occur  within  the  parent  igneous  rock  they  are  usually  coarse- 
grained aggregates  of  the  same  constituents  as  those  found  in  the 
main  mass.  They  represent  a  continued  growth  of  the  individual 


POST-VOLCANIC  PROCESSES  143 

constituents,  and  the  boundary  between  the  two  rocks  is  blended 
and  indistinct.  This,  and  the  irregular  shape  of  the  intrusions, 
show  that  the  pegmatites  were  formed  during  the  later  stages  of  the 
solidification  of  the  main  mass,  and  that,  with  the  aplites,  they 
represent  the  first  extrusions  after  the  original  igneous  activity. 

Pegmatites  related  to  each  type  of  plutonic  rock  are  known. 
They  are  most  commonly  and  most  abundantly  developed  in 
connection  with  granites  and  nephelite-syenites,  but  are  relatively 
rare  with  plagioclase  rocks.  The  more  basic  the  plutonic  rock,  the 
simpler  are  its  satellites.  The  pegmatites  normally  have  aplitic 
fades,  and  in  places  pass  through  all  possible  transitions  to 
aplites.  But  lamprophyres  also,  here  and  there,  have  pegmatitic 
habits.  Coarse-grained  portions  of  kersantite  dikes,  and  some 
pegmatite-like  nephelinite  schlieren  in  basalts,  show  undoubted 
affinities  with  true  pegmatites. 

Pegmatites  have  coarse  to  very  coarse  textures,  and  may  vary 
greatly  both  in  texture  and  composition.  Eutectic  mixtures, 
causing  parallel  intergrowths  of  the  individual  minerals,  are  espe- 
cially widespread,  for  example  in  graphic  granite  or  pegmatite  in 
the  narrow  sense.  The  development  of  gigantic  crystals  and  of 
crystal-druses  suggest  the  origin  of  these  rocks  from  magmas 
especially  rich  in  mineralizers.  This  mode  of  origin  seems  a 
certainty  after  a  close  examination  of  the  minerals  themselves  and 
of  the  intense  solvent  power  which  the  melt  had  upon  the  con- 
stituents of  the  country-rock.  In  some  cases  this  assimilation  of 
foreign  constituents  was  so  great  that  it  altered  the  entire  character 
of  the  rock. 

A  careful  examination  of  the  minerals  of  the  pegmatites  shows  that  there  must 
have  been  a  remarkable  concentration  of  mineralizers,  and  that  the  so-called  rare 
elements,  which  are  present  in  normal  rocks  only  in  traces,  are  very  abundant. 

The  minerals  of  the  pegmatites  may  be  grouped  as  follows: 

1.  The  minerals  of  the  parent  rock,  namely:  quartz,  orthoclase,  albite,  and  white 
and  more  rarely  dark  mica;  and  in  certain  rocks,  anorthoclase,  nephelite,  sodalite, 
etc.,  with  segirite  and  arfvedsonite. 

2.  Minerals  containing  especially  active  elements:  tourmaline,  topaz,  fluorite, 
scapolite,  and  apatite. 

3.  Minerals  containing  rare  elements :  monazite,  xenotime,  orthite,  beryl,  chryso- 
beryl,  niobates  and  tantalates,  molybdenite,  zircon,  titanite,  and,  in  nephelite-syenite 
pegmatites,  the  zirconium  silicates,  especially  lavenite,  mosandrite,  rinkite,  astrophyl- 
lite,  katapleite,  etc. 

4.  Minerals  derived  from  the  country-rocks:  andalusite,  disthene,  garnet,  cordier- 
ite,  staurolite,  and  the  like. 

10 


144         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

To  these  must  be  added  occasional  ore-minerals,  and,  as  final  products  showing 
the  gradual  transition  into  the  thermal  stage,  various  zeolites  and  some  opal. 

This  extraordinary  paragenesis  occurs  only  in  the  alkali-rich  rocks.  The  peg- 
matites of  the  plagioclase  series  are  much  simpler,  in  many  cases  being  merely  coarsely 
developed  phases  of  the  parent  rock.  Crystal-druses  are  usually  very  abundant  in 
granite-,  syenite-,  and  nephelite-syenite-pegmatites,  and  there  may  be  a  remarkable 
development  of  beautifully  crystallized  minerals;  in  the  plagioclase-pegmatites, 
druses  are  rarely  present. 

Pegmatites  rich  in  tourmaline,  scapolite,  etc.,  may  impregnate  the  country-rock 
with  these  minerals  to  a  considerable  distance.  Ordinarily,  however,  the  minerals 
developed  depend  to  a  large  extent  upon  the  composition  of  the  country-rock.  For 
example,  where  the  granite-pegmatites  of  the  Bavarian  Forest  pass  into  the  aluminium- 
rich  contact-rocks,  large  druses  containing  andalusite  are  developed.  In  the  Fichtel- 
gebirge  a  similar  rock  cuts  eclogite  and  becomes  a  coarse-grained  aggregate  of  feldspar 
and  zoisite.  In  other  regions  the  constituents  dissolved  from  the  country-rock 
form  staurolite,  garnet,  disthene,  etc.  The  orthoclase  content  of  these  dikes  is  readily 
lost  and  albite  takes  its  place,  in  many  cases  becoming  an  important  constituent. 

That  there  is  a  connection  between  the  pegmatites  and  vulcanism  has  been  denied 
on  the  strength  of  these  abnormal  relationships,  and  these  rocks  have  been  ascribed 
to  lateral  secretion,  that  is  to  the  leaching  of  the  country-rock  by  circulating  water. 
The  manner  of  their  occurrence,  however,  is  decidedly  against  this  view,  and  their 
intimate  connection  with  igneous  rocks  is  too  apparent.  Furthermore,  they  do  not 
bear  the  slightest  resemblance  in  texture  or  composition  to  deposits  from  vadose 
solutions. 

Further  modifications  may  be  produced  in  the  pegmatites  by  the  action  of  piezo- 
crystallization.  In  many  cases  masses  of  alkali-mica  scales  take  the  place  of  the  feld- 
spars, and  normal  pegmatite  dikes,  which  extend  for  long  distances,  may  grade  into 
schistose  aggregates  of  mica  closely  resembling  mica-schists.  The  drusy  texture  is 
then  usually  entirely  wanting.  Such  modified  pegmatites,  carrying  staurolite  and 
disthene  derived  from  the  contact-metamorphosed  country-rock,  occur  in  the  Tessin 
(Ticino)  paragonite-schists.  On  the  other  hand,  perfectly  normal  pegmatites  are 
found  in  the  central  granite.  Like  the  mineral-rich  dikes  of  the  Titan-formation, 
which  cut  similar  rocks,  they  contain  innumerable  druses  filled  with  magnificent 
crystals.  This  drusy  condition  indicates  a  cessation  of  pressure  during  their  formation ; 
in  fact,  the  dikes  intruded  immediately  after  the  main  granite  of  the  central  Alps 
show  in  many  places  that  orographic  forces  were  absent. 

The  Amygdaloids. — Vesicular  and  scoriacious  rocks,  with  their 
cavities  entirely  or  partially  filled  with  secondary  minerals,  are 
widely  distributed  among  dike-rocks,  especially  among  lampro- 
phyres.  They  also  occur  among  the  extrusives,  and  are  as  com- 
mon among  basic  rocks  as  among  silicic.  Such  rocks  (Fig.  86) 
are  called  amygdcdoids  (Gr.  d,uuy 5dXr7,  almond) .  While  their  charac- 
teristic paragenesis  has  usually  been  ascribed  to  lateral  secretion, 
whereby  the  mineral-matter  is  supposed  to  have  been  deposited 
from  solutions  leached  by  atmospheric  agents  from  the  country- 
rocks,  as  a  matter  of  fact  these  minerals  are  predominantly  those 
which  elsewhere  are  universally  associated  with  igneous  rocks,  and 
which  have  never  been  known  to  be  deposited  from  vadose  waters. 


POST-VOLCANIC  PROCESSES 


145 


Among  the  amygdule  fillings,  chalcedony  and  the  zeolites  are  especially  interesting. 
These  minerals  are  elsewhere  known  only  in  connection  with  hot  juvenile  springs,  and 
therefore  probably  have  a  similar  mode  of  origin  where  they  fill  the  vesicules.  Most 
typical  occurrences  are  agate-amygdules.  These  vary  in  size  from  a  few  millimeters 
to  over  a  meter,  and  their  banding  (Fig.  87)  clearly  shows  the  gradual  filling  of  the 
cavities.  Various  features  suggest  that  the  chalcedony  was  originally  gelatinous 
and  later  became  crystalline.  In  many  cases  the  agate-amygdaloids  are  hollow  and 
lined  with  quartz  crystals,  commonly  amethyst,  a  form  of  quartz  which  is  never  of 
unquestionable  vadose  origin.  Among  other  minerals  occurring  in  this  manner  are 
calcite  and  aragonite,  various  zeolites,  here  especially  well  developed,  prehnite, 
boron-bearing  datolite,  and,  in  certain  melaphyres,  abundant  native  copper.  Scaly 
hematite  and  goethite  are  also  present.  Further,  green  substances  of  various  kinds, 
such  as  seladonite  or  chloritic  minerals,  are  abundant  in  the  vesicules,  and  with  them, 
in  many  cases,  epidote  or  different  colloidal  silicates.  These  minerals  are  found 
not  only  in  the  vesicules  of  the  lava-streams  but  also  in  the  larger  cavities  of  bombs 
lying  in  volcanic  tuffs,  and  here  and  there  in  fissures. 


FIG.  86. — Melaphyre  amygdaloids. 
Oberstein  a.  N. 


FIG.  87. — Agate  showing  conduit. 
Oberstein  a.  N. 


The  vesicular  character  of  the  lava  is  doubtless  primary  and  due  to  the  escape  of 
the  gaseous  mineralizers  during  the  solidification  of  the  rock.  Besides  these  rounded 
vesicules,  however,  there  are  found  in  the  effusive  rocks  many  other  cavities  which  are 
of  irregular  shapes  and  appear  to  be  due  to  corrosion.  These  cavities  probably  origi- 
nated in  the  dissolving  action  of  gases  during  the  last  stages  of  the  solidification  of 
the  magma,  and  in  them,  minerals  of  an  entirely  different  kind  are  deposited.  Thus, 
tridymite  and  topaz  are  found  in  rhj^olites  and  trachytes,  while  well-developed 
crystals  of  sanidine,  nephelite,  sodalite,  leucite,  melilite,  etc.,  as  well  as  olivine,  pyrox- 
ene, amphibole,  and  locally  calcium-magnesium  garnets,  and  even  the  rare  lievrite, 
occur  in  the  soda-rocks.  Probably  all  of  these  minerals  are  of  pneumatolytic  origin, 
and  water  had  little  to  do  with  their  formation.  The  local  occurrence  of  opal,  however, 
shows  that  pneumatolysis  was  sometimes  combined  with  the  hydration  of  the  thermal 
period. 

Finally,  there  remain  to  be  mentioned  certain  peculiar,  mostly  colloidal  phosphates 
which  occur  predominantly  in  fissures  of  volcanic  rocks,  and  which  unquestionably 
also  belong  to  the  post-volcanic  thermal  period.  The  turquois  deposits  in  trachytes 
and  trachyte  tuffs  of  Persia  and  the  amorphous  phosphorite  films  on  the  parting- 
planes  of  many  basaltic  columns  are  of  this  kind. 


146         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

Mineral-dikes  and  Ore -veins. — By  far  the  greater  number  of 
fissure  deposits  of  minerals  and  ores  are  of  juvenile  origin,  and 
while  certain  ore  deposits  may  be  far  removed  from  any  igneous 
rocks,  an  examination  of  their  general  relationships  very  rarely 
shows  them  to  be  of  vadose  origin.  Most  mineral-dikes  and 
ore-veins  are  of  less  importance  in  petrology  than  they  are  in 
mineralogy  and  economic  geology,  therefore  only  a  few  especially 
typical  examples  will  be  cited. 

Tin  deposits  are  the  most  noteworthy  and  typical  representatives  of  the  pneu- 
matolytic  period  of  volcanic  activity,  a  period  characterized  especially  by  mobility 
of  gases  under  high  pressure.  These  gases,  emanating  from  the  magma  and  loaded 
with  powerful  mineralizers,  penetrated  fissures  and  dikes,  and  generally  saturated  the 
country-rocks  to  great  distances.  The  ore  deposits  are  genetically  connected  with 


Graphite 
lenses 


Gneiss 


FIG.  88. — Occurrence  of  graphite  lenses  in  the  neighborhood  of  Passau. 


granitic  rocks  which  either  contain  a  very  small  amount  of  tin  and  other  minerals 
of  the  dikes  (tin-granite),  or  whose  mica  carries  lithium  (lepidolite-granite  or  its 
porphyritic  equivalent,  lithium-bearing  quartz-porphyry). 

The  nature  of  these  ore-bearing  dikes  is  well  shown  by  the  extent  of  the  mineraliza- 
tion of  the  country-rocks,  for  whether  the  walls  are  granite,  granite-porphyry,  or 
quartz-porphyry,  or  injection-schists  or  other  contact-rocks,  they  are  impregnated 
to  a  considerable  distance  by  quartz,  tourmaline,  topaz,  fluorite,  and  other  minerals 
of  the  tin  formation.  Most  noteworthy  is  the  fact  that  the  feldspar  of  the  original 
granite  has  disappeared,  and  topaz-  and  fluorite-rich  greisen  is  formed.  In  the 
granite-porphyry  called  luxullianite,  the  groundmass  is  changed  to  a  dark,  tourmaline- 
quartz  aggregate,  the  larger  feldspars  either  remaining  unaltered  or  becoming  changed 
to  tourmaline,  topaz,  or  tin  ore.  Many  rocks  have  been  altered  to  tourmaline-  or 
topaz-rich  quartzites  (zwitters),  while  the  entire  feldspar  content  of  others  has  become 
kaolinized.  All  rocks,  irrespective  of  their  original  character,  show  alterations  of 
this  kind,  and  everywhere  near  the  dikes,  cassiterite  and  its  constant  companion, 
arsenopyrite,  occur  as  impregnations.  Farther  from  the  dike  the  normal,  unaltered 
rock  appears. 

Among  ore-veins  none  furnishes  a  stronger  argument  against  the  lateral  secre- 


POST-VOLCANIC  PROCESSES  147 

tion  theory  than  does  cassiterite.  Here,  contrary  to  the  theory,  the  country-rock 
has  in  many  cases  been  altered  by  the  action  of  true  fumaroles  to  a  considerable  dis- 
tance from  the  dike,  and  the  ore  formation  was  everywhere  accompanied  by  the  forma- 
tion of  tourmaline  and  topaz,  which  were  no  more  deposited  from  vadose  circulating 
solutions  than  was  the  cassiterite  itself.  The  phenomena  connected  with  tourmaline- 
bearing  copper-ores  are  analogous,  and  zwitter  and  greisen  are  developed,  although 
to  a  less  extent. 

The  effects  of  post-volcanic  processes  are  very  characteristically  shown  by  certain 
graphite  deposits,  especially  by  those  occurring  in  dikes  in  the  granulite  of  Ceylon, 
and  by  the  graphite-gneiss  which  is  found  in  small  lenses  in  injection-schists  near 
the  granite-contact  at  Passau,  Bavaria  (Fig.  88).  In  the  latter  especially,  the  action 
of  volcanic  agencies  is  very  clear.  The  parts  rich  in  graphite  represent  impregnations 
of  the  injection-schists,  for  wherever  graphite  occurs  they  are  more  or  less  completely 
altered  to  kaolin,  nontronite,  and  amorphous  silicates  of  manganese,  and  in  many 
cases  are  impregnated  with  opal. 

Injection-schists  are  ordinarily  very  compact  rocks  and  form  steep  cliffs,  but  in 
the  graphite  area  they  are  altered  to  unconsolidated  earth.  On  the  other  hand,  the 
younger  lamprophyre  and  aplite  dikes,  which  cut  and  fault  the  graphite  lenses,  have 


a    a     a    a  .a    a    a   a 


Gabbro  Scapolite-gabbro  Apatite  dikes 

FIG.  89. — Apatite  dikes  with  scapolitized  gabbro.     Husaas,  Norway.     (After] 

J.  H.  L.  Vogt.) 

remained  unaltered  in  spite  of  their  high  pyrite  content.  There  can  be  no  question 
as  to  the  secondary  character  of  the  graphite  in  the  injection-schists,  nor  of  the  intimate 
relation  between  the  intense  metamorphism  and  the  formation  of  the  graphite. 
The  large  amounts  of  iron  and  manganese  which  are  present  can  only  have  been 
brought  in  with  the  graphite,  for  since  these  metals  occur  in  their  highest  states  of 
oxydation  they  cannot  be  ascribed  to  reducing  agents.  Probably  most  of  the  graphite 
was  brought  in  as  unstable  carbon  compounds,  which  easily  broke  up  into  carbon 
dioxide  and  oxides  of  the  metals.  At  any  rate,  these  graphite  deposits  are  due  to  post- 
volcanic  processes. 

The  alterations  in  the  country-rock  around  the  rutile-apatite  dikes  of  South  Nor- 
way also  point  to  intensive  post-volcanic  processes.  The  country-rock  is  a  normal 
gabbro,  but  near  the  dikes  it  is  impregnated  with  chlor-scapolite,  and  in  many  cases 
is  entirely  altered  to  a  hornblende-scapolite  rock  filled  with  rutile  and  apatite  (Fig.89). 

The  central  Alpine  Titan  formation  is  somewhat  different  in  appearance,  but  it 
is  very  intimately  related  to  the  pegmatites  of  the  central  granite.  Here  the  country- 
rocks  had  a  great  influence  upon  the  dikes.  This  influence  can  be  explained  only 
on  the  assumption  that  hot  gases  had  less  to  do  in  changing  the  character  of  the  dikes 
than  had  heated  solutions,  which  dissolved  considerable  material  from  the  surround- 
ing rocks.  The  dikes  are  characterized  by  the  constant  presence  of  titanium  oxide, 


148         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

although  this  does  not  occur  in  quantities  great  enough  to  be  of  commercial  impor- 
tance. Where  the  dikes  cut  the  granite,  the  chief  constituents  are  quartz  and  adula- 
ria,  with  rutile,  anatase,  and  brookite.  In  many  cases  these  minerals  form  beautiful 
crystals,  and  the  so-called  Crystal  Cellar  of  the  Alps  occurs  in  this  part  of  the  formation. 
Toward  the  contact-zone,  prehnite,  zeolites,  and  titanite,  and  farther  on  diopside, 
zoisite,  and  epidote  occur  in  the  dikes.  Where  they  pass  into  amphibolite  or  green- 
stone-schist the  latter  three  minerals  become  the  chief  constituents,  the  quartz  disap- 
pears, and  in  place  of  adularia,  beautiful  crystallized  albite  is  developed.  In  many 
places  minerals  characteristic  of  the  pegmatite  itself  also  appear,  among  them  are 
monazite,  beryllium-bearing  euclase,  tourmaline,  some  fluorite  and  apatite,  ore- 
minerals  of  many  kinds,  and  even  native  gold.  The  dikes  become  still  richer  in 
minerals  where  they  traverse  any  of  the  numerous  serpentine  stocks  of  the  region, 


Andesite  Ore  veins  Propylite 

FIG.  90. — Sketch  map  of  the  neighborhood  of  Schemnitz,  Hungary. 


as  was  shown  on  page  133.     The  influence  of  the  intrusives  upon  the  country-rock, 
on  the  other  hand,  is  at  most  very  slight. 

Opposed  to  these  processes,  in  a  certain  way,  are  the  replacement  processes  in 
the  dikes  of  the  so-called  propylitized  gold-silver  formation.  Various  igneous  rocks, 
chiefly  andesites,  are  impregnated  with  pyrite  adjacent  to  the  ore  deposits.  They 
are  also  otherwise  altered  to  greater  distances,  yet  never  outside  the  zone  of  influence 
of  the  dike  itself  (Fig..  90).  This  alteration  process  is  called  propylitization,  and  by 
it  the  original  anhydrous  minerals  of  the  igneous  rocks  were  altered  to  hydrous  min- 
erals such  as  chlorite,  sericite,  kaolin,  etc.  Propylite  (Gr.  irpoiniXop,  entrance,  since 
the  rocks  were  supposed  to  be  the  earliest  extrusives  of  the  Tertiary  igneous  cycle) 
was  formerly  supposed  to  be  an  independent  kind  of  rock,  intermediate  between  the 
older  and  younger  igneous  rocks.  It  actually  is  only  an  altered  variety  of  andesite, 
and  perfectly  fresh  rock  occurs  farther  from  the  dikes.  Propylitization  is  a  form  of 
pneumatohydatogenic  alteration  in  which  the  country-rock  had  no  influence  upon  the 
character  of  the  dike. 


POST-VOLCANIC  PROCESSES  149 

Varieties  of  Rock  Alteration. — The  replacement  phenomena 
associated  with  these  secondary  formations  are  transitional  to 
true  post-volcanic  metamorphism.  The  characteristics  of  the 
latter  as  a  deep-seated  process,  and  the  differences  between  it  and 
weathering  have  been  mentioned  previously.  The  local  causes 
for  replacement  are  very  variable,  as  is  also  the  ability  of  the 
different  minerals  to  resist  the  different  processes,  consequently 
the  course  of  the  reaction  itself  and  the  results  to  which  it  leads  are 
also  variable.  Minerals  which  can  best  withstand  weathering 
are  in  some  cases  easily  destroyed  by  replacement,  and  the  contrary 
is  just  as  commonly  true.  Tourmaline  and  disthene,  for  example, 
may  be  altered  to  mica-like  substances  by  post-volcanic  agents, 
but  they  are  entirely  unaffected  by  simple  weathering.  Again, 
biotite,  monazite,  and  xenotime  are  absolutely  fresh  in  completely 
kaolinized  granites,  yet  they  are  very  readily  decomposed  by 
weathering. 

The  most  important  post-volcanic  metamorphic  processes  are 
the  following: 

1.  Kaolinization.  Kaolinization  is  probably  one  of  the  most 
characteristic  of  replacement  processes,  and  is  of  rather  widespread 
occurrence.  It  is  of  most  importance  in  granites  and  quartz- 
porphyries,  but  takes  place  in  other  rocks,  and  is  even  present  in 
very  basic  varieties.  The  alteration  normally  appears  in  isolated, 
larger  or  smaller  patches,  or  in  a  series  of  such  patches  along  a 
fissure.  It  differs  from  weathering  in  that  it  primarily  attacks 
the  feldspar,  acting  upon  plagioclase  more  readily  than  upon 
orthoclase,  but  scarcely  affecting  microcline. 

Weathering  changes  granite  into  rusty  grush  in  which  the  alkali  content  remains 
high,  but  kaolinization  completely  removes  the  alkali  as  well  as  the  lime  from 
both  plagioclase  and  orthoclase.  Biotite  is  usually  the  first  mineral  to  be  affected 
by  weathering,  but  it  is  fresh  in  many  kaolinized  rocks.  It  is  especially  characteristic 
that  apatite,  which  is  unaffected  by  chemical  weathering,  disappears  completely 
under  the  action  of  kaolinization,  and  monazite  and  xenotime,  which  become  cloudy 
upon  the  slightest  weathering,  are  always  clear  and  fresh  in  kaolin. 

The  kaolinized  patches  in  granite,  even  directly  at  the  surface,  laterally  pass 
abruptly  into  normal  rock.  In  depth,  on  the  other  hand,  and  this  must  be  especially 
emphasized  on  account  of  innumerable  assertions  to  the  contrary,  such  a  transition 
can  nowhere  be  seen.  It  is  true  that  drill-holes  may  show  compact  granite  below  the 
kaolin,  but  this  is  probably  due  to  the  irregular  form  of  the  kaolinized  patches,  as 
is  shown  in  Fig.  91,  in  which  the  broken  lines  represent  three  drill-holes.  Where  the 
degree  of  kaolinization  alters,  it  invariably  increases  with  depth,  even  to  400  to. 500 
meters,  where  atmospheric  agents  must  of  course  be  absent.  Nests  of  tourmaline 
are  not  uncommon  in  kaolin  deposits,  and  the  purest  kaolin  occurs  in  their  vicinity. 


150 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


Furthermore,  mechanical  rock  analyses  show  the  presence  of  a  small  quantity^of 
tourmaline,  topaz,  fluorite,  pyrite,  and  siderite  in  numerous  kaolin  deposits,  and  these 
minerals,  which  do  not  occur  in  the  original  rocks,  indicate  the  nature  of  the  agency 
which  produced  the  change. 


Granite  Kaolin 

FIG.  91. — Ideal  section  through  a  kaolin  deposit  in  granite. 

Many  kaolinized  rocks  are  associated  with  deposits  produced  by  pneumatolysis, 
a  process  which  always  produces  intense  alteration  in  the  accompanying  rocks.  Thus 
propylite,  and  rocks  within  the  sphere  of  influence  of  some  graphite  deposits  or  cassit- 
erite  dikes,  are  altered  to  kaolin  in  the  last  stage  of  replacement.  Furthermore, 


Faults 


KARLSBAD 
\  :['  Sprudel 


FIG.  92. — Sketch  map  of  the  neighborhood  of  Carlsbad.     Shows  the  relationship 
between  the  kaolinized  patches  and  the  fault  lines. 

near  many  kaolin  deposits,  as  at  Karlsbad  in  Bohemia  (Fig.  92),  there  are  active  hot 
springs  even  at  the  present  time,  and  the  relationship  between  the  two  is  very  clear. 
The  fact  that  pegmatite  and  aplite  are  kaolinized  in  so  many  cases  and  so  intensely 
shows  the  activity  of  the  gases  and  vapors  producing  this  type  of  alteration. 


POST-VOLCANIC  PROCESSES  151 

2.  Saussuritization,  uralitization,  and  the  formation  of  green- 
stones, are  alteration  phenomena  in  basic  igneous  rocks,  but,  as 
was  shown  on  page  131,  they  may  also  appear  in  granite  itself  as 
the  result  of  contact-metamorphism.  Alteration  may  take  place, 
however,  without  the  influence  of  foreign  plutonic  rocks,  as  was 
shown  under  the  discussion  of  propylite,  a  rock  belonging  to  the 
greenstone  series.  Under  such  metamorphism,  the  original  tex- 
ture of  the  rock  generally  remains  quite  distinct.  The  feldspar,  in 
some  cases,  is  altered  to  a  dense  aggregate  of  saussurite,  and  such 
greenish,  saussuritized  porphyrites  and  other  similar  rocks  are 
among  the  toughest  and  most  resistant  of  rocks.  Besides  the 
calcium-aluminium  silicates  which  compose  the  saussurite  sericite 
and  calcite  may  occur,  and  in  some  cases,  as  in  normal  greenstones, 
may  predominate  in  amount.  In  such  cases,  the  pyroxene  is 
rarely  truly  uralitized,  but  is  changed  to  an  aggregate  consisting 
chiefly  of  chlorite  with  some  epidote  and  perhaps  a  little  uralite. 
Primarily  the  process  is  one  of  hydration  and  decalcification. 
An  impregnation  by  pyrite  is  always  associated  with  the  formation 
of  greenstone,  and  pyrite  is  probably  always  secondary  in 
saussuritized  and  uralitized  rocks. 

Greenstones  are  most  commonly  formed  from  silica-poor  roeks.  They  are  rarely 
formed  from  those  that  are  rich  in  silica,  and  apparently  never  from  sodic  rocks. 
Dikes  and  extrusives  are  most  subject  to  this  alteration;  among  the  former,  lampro- 
phyres,  and  among  the  latter,  andesite  and  porphyrite,  trap,  and  melaphyre  are  espe- 
cially susceptible. 

Two  entirely  different  types  of  alteration  may  be  distinguished  among  these  rocks. 
In  the  first  type,  the  rocks,  which  are  gray  or  brownish  black  when  fresh,  become 
brown,  brownish  red,  or  red.  These  are  the  usual  colors  of  altered  quartz-porphyries, 
and  they  are  not  uncommon  among  basic  porphyrites  and  melaphyres.  The  origin- 
ally compact  rock  eventually  acquires  an  uneven  fracture  and  loses  its  luster,  and  its 
marked  clay-like  odor  has  given  rise  to  the  name  clay-stone  porphyry.  Finally, 
the  rocks  take  on  a  dull  uniform  color,  and  form  masses  which  are  distinguishable 
with  difficulty  from  each  other.  They  were  called  wacke  by  the  earlier  geologists. 
The  final  phase  of  this  alteration  process  makes  the  rock  character  unrecognizable, 
and  is  doubtless  due  to  atmospheric  agents.  Whether  the  beginning  of  the  alteration, 
which  is  marked  by  an  intense  impregnation  with  iron  oxide,  is  due  to  the  same 
cause,  must  still  be  considered  questionable. 

The  alteration  shown  by  greenstone-porphyry  and  diabase  is  quite  different. 
Here  the  entire  process  was  doubtless  one  of  true  replacement  by  means  of  hot  juvenile 
waters,  which  were  probably  always  sulphur  bearing,  as  seems  to  be  indicated  by  the 
constant  presence  of  pyrite  in  rocks  so  altered. 

The  formation  of  saussurite  has  often  been  considered  typical  of  dynamometamor- 
phism  on  account  of  the  relatively  high  specific  gravity  of  the  calcium-aluminium 
silicates  which  are  present  in  it.  As  was  shown  above,  most  saussuritized  rocks  result 
from  contact-metamorphism.  The  alteration  is  not  necessarily  connected  with 


152          FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

especially  intense  tectonic  processes,  but  depends  upon  causes  similar  to  those  produc- 
ing greenstones,  the  difference  being  due  perhaps  to  greater  depth  and  pressure  during 
the  process  of  replacement.  More  hypabyssal  and  plutonic  rocks  than  extrusives, 
therefore,  are  saussuritized,  while  there  are  more  greenstones  derived  from  extrusives. 

3.  Sericitization  in  acid  rocks  corresponds  somewhat  to  the 
formation  of  greenstone  from  those  that  are  basic.     Sericitized 
aggregates  derived  from  feldspars  occur  even  in  the  latter,  while 
chlorite  is  widely  distributed  in  sericitized  granites  or  quartz- 
porphyries.     Sericite  occurs  locally  in  the  Erzgebirge  at  the  con- 
tact between  granites,  gneisses,  and  quartz-porphyries  and  veins 
of  silver-bearing  galena,  the  formation  of  the  latter  having  pro- 
duced the  alteration  in  the  'former.     It  also  occurs  where  rocks 
show  considerable  faulting,  for  example  in  the  Bavarian  Forest 
where  the  granite  is  altered  to  the  Pfahl  schist,  a  rock  consisting 
of  finely  brecciated  quartz  separated  by  fine  sericite  films. 

The  micaceous  character  of  the  alteration  is  especially  dis- 
tinct in  the  fine,  silky-lustered,  light-colored  sericite-schists  pro- 
duced from  crushed  quartz-porphyries.  In  these  rocks,  which  in 
many  cases  have  paper-thin  schistosity,  the  rounded  or  embayed 
quartz  phenocrysts  of  the  former  quartz-porphyry  may  still  be 
recognized,  and  they  may  even  be  visible  megascopically  as  small, 
hard  knots  on  cleavage  surfaces  of  the  crinkled  rock. 

Sericitization  also  has  frequently  been  considered  a  dynamometamorphic  change, 
and  it  is  undoubtedly  true  that,  in  numerous  cases,  mechanical  forces  have  assisted  in 
the  alteration.  Probably  the  most  characteristic  examples  are  offered  by  the  Pfahl 
schists  and  certain  poryhyroids  occurring  in  the  strongly  folded  Paleozoic  rocks  of 
the  Taunus  and  the  Ardennes,  the  latter  having  been  altered  to  sericite-schists  where 
mashed  to  an  exceptional  degree.  That  thermal  activity  was  here  of  great  importance 
is  shown  by  the  enormous  quartz-dikes  (and  their  accompanying  minerals)  which  are 
associated  with  the  Pfahl  schists  and  with  other  closely  related  alteration  products 
of  the  crushed  granite.  In  other  cases,  however,  Sericitization  has  altered  quartz- 
porphyry  to  a  silky-lustered,  white  schist  without  producing  any  observable  deforma- 
tion in  the  quartz  phenocrysts.  Many  such  schists  are  interbedded  with  contact- 
rocks,  and  it  is  probable  that  the  Sericitization  of  the  quartz-porphyry  is  here  due  to 
contact-metamorphism.  It  is  hardly  necessary  to  say  that  quartz-porphyry-tuffs 
under  analogous  conditions  are  similarly  altered,  so  that  after  metamorphism  the 
igneous  rock  cannot  be  separated  from  its  tuff. 

4.  Serpentinization.     The  formation  of  serpentine  from  olivine- 
rocks  may  be  traced  in  most  cases  to  the  action  of  contact-meta- 
morphism, as  described  on  page  132.     Some  serpentines,  however, 
may  have  originated  from  originally  anhydrous  olivine-rocks  by 
thermal  processes  following  their  intrusion,  just  as  certain  green- 
stones   originated    by    a    similar   process.     Nevertheless,    large 


POST-VOLCANIC  PROCESSES  153 

mountain-making  masses  of  serpentine  are  certainly  not  products 
of  normal  weathering,  and  just  as  little  are  they  the  results  of 
dynamometamorphism.  The  attempt  has  been  made  to  assign 
different  modes  of  origin  to  the  two  varieties  of  serpentine;  antig- 
orite,  which  occurs  principally  in  folded  mountain  regions,  is 
regarded  by  some  as  the  result  of  the  erogenic  forces,  while 
chrysotile  is  assumed  to  be  the  normal  product  of  weathering. 

Serpentine  [(Mg,Fe)O  :  SiO2  =  3:2]  may  be  regarded  as  consisting  of  one  part 
olivine  [(Mg,Fe)O  :  SiO2  =  2:1]  and  one  part  bronzite  [(Mg,Fe)O  :  SiO2  =  1:1], 
but  it  is  doubtful  if  olivine  and  bronzite  in  these  proportions  ever  resulted  in  the 
formation  of  serpentine.  Most  serpentines  originated  from  peridotites,  that  is, 
from  rocks  in  which  olivine  greatly  pre- 
dominated, and  this  fact  alone  shows  that 
the  formation  of  serpentine  is  a  more  com- 
plicated process  than  is  usually  supposed 
since  a  considerable  amount  of  silica  must 
be  added  or  great  amounts  of  magnesia  and 
iron  subtracted.  Furthermore,  the  develop- 
ment of  other  new  minerals,  such  as  talc, 
actinolite,  chlorite,  and  carbonates,  indi- 
cates that  the  process  is  not  simple. 

With  the  exception  of  a  few  unimpor- 
tant contact-metamorphosed  carbonate- 
rocks,  all  serpentines  were  derived  from 
rocks  originally  igneous.  In  innumerable 
cases  they  accompany  greenstones  and 

greenstone-schists,   which   are  undoubtedly 

&.  .  ,    ,  FIG.    93. — Serpentimzed    olivine 

of  igneous  origin,  and  they  are  widespread     crystals  in  picrite.     Trogen,  near  Hof , 
as  dikes.     It  is  true  that  serpentines  gener-     Fichtelgebirge. 
ally  kform  lens-shaped  beds  intercalated  be- 
tween other  rocks,  but  where  they  are  interbedded  with  limestones  and  marls,  they 
are  surrounded  in  many  cases  by  normal  contact-rocks. 

Like  the  accompanying  greenstone  and  saussuritefels,  serpentine  is  probably 
never  an  original  rock.  Although  in  some  cases  it  has  been  considered  original, 
microscopic  examinations  almost  invariably  show,  by  the  more  or  less  abundant 
remnants  of  the  anhydrous  silicate  still  present,  or  by  the  mesh  texture  of  the 
chrysotile  or  the  grating  texture  of  the  antigorite,  that  olivine  was  the  chief  original 
constituent. 

Where  olivine  occurred  as  a  subordinate  constituent  in  such  rocks  as  gabbros, 
traps,  melaphyres,  and  basalts  which  have  been  serpentinized,  its  outlines  are  still 
well  preserved.  In  true  serpentine  masses  such  well-bounded  pseudomorphs  are 
wanting,  the  original  rock  having  consisted  originally  of  an  equigranular  aggregate  of 
olivine.  Porphyritic  olivine-rocks  are  extremely  rare,  but  where,  they  do  occur,  the 
phenocrysts  with  their  mesh  structure  and  the  preserved  remnants  of  the  original 
mineral,  stand  out  distinctly  from  an  extremely  'dense  serpentine  groundmass 
(Fig.  93). 

5.  The  formation  of  talc  must  be  considered  here  as  an  appendix 
to  serpentinization,  since  talc  may  be  produced  by  this  process  as 


154         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

an  accessory  mineral.  Talc  differs  from  serpentine,  which  is  de- 
rived almost  exclusively  from  olivine,  in  being  an  alteration  prod- 
uct of  many  minerals,  magnesia-bearing  and  magnesia-free,  silicate 
and  non-silicate.  It  may  be  formed  from  olivine-rocks  as  well  as 
from  dolomites,  from  limestones  as  well  as  from  granites.  Even 
true  slates  may  be  re-formed  into  pure  talc-rocks.  This  most  rad- 
ical replacement  process  uniformly  attacks  everything  that  comes 
within  reach  of  the  re-forming  magnesia-rich  solutions,  and  is 
rather  widespread  in  the  contact- zones  of  granites,  where  such 
solutions  are  always  strikingly  abundant.  Important  deposits 
of  compact  talc  which  were  formed  in  this  manner  in  all  probability 
originally  consisted  of  the  colloidal  alteration  products  of  various 
minerals  and  rocks  at  the  granite  contact.  Where  talc-rocks  occur 
as  patches  and  dikes  in  serpentine,  they  are  more  distinctly 
crystalline  than  they  are  elsewhere. 

In  many  cases  much  chlorite  is  developed  simultaneously  with  the  talc,  as  in  soap- 
stone,  which  occurs  in  granite  as  well  as  in  serpentine.  Furthermore,  many  of  the 
granites  in  the  neighborhood  of  the  talc  beds  of  the  Fichtelgebirge  are  intensely 
chloritized,  and  pseudomorphs  of  chlorite  after  orthoclase  are  not  uncommon.  Fi- 
nally, in  completely  mashed  granites,  such  as  the  so-called  Winzer  granite  on  the  banks 
of  the  Danube  east  of  Regensburg,  the  entire  feldspar  content  is  in  many  cases  altered 
to  a  dense  aggregate  of  chlorite. 

6.  Zeolitization  is  a  replacement  process  which  primarily  attacks 
nephelite,  leucite,  and  minerals  of  the  sodalite  group.  That  this 
alteration  is  also  due  to  thermal  action  is  shown  by  the  fact  that 
zeolitized  and  perfectly  fresh  rocks  occur  side  by  side  in  places 
where  the  hydrographic  conditions  are  the  same. 

The  phonolite  domes  of  Hohentwiel  in  Hegau,  which  are  especially  well  known 
because  they  carry  natrolite,  are  penetrated  by  a  well  to  a  depth  of  100  meters, 
and  all  of  the  rocks  taken  from  it  are  uniformly  zeolitized,  yet  very  small  ejected 
fragments  of  the  same  rock  in  the  surrounding  tuffs  contain  no  zeolites,  even  at  the 
surface,  but  instead  carry  considerable  amounts  of  perfectly  fresh  hauynite  and 
noselite. 

7.  Besides  these  ordinary  phenomena  of  post-volcanic  metamorphism,  others 
of  purely  local  character  should  be  mentioned,  such  as  the  alteration  of  rhyolites 
and  rhyolite  tuffs  to  alunite,  and  the  formation  of  potash-rich  seladonite  in  potash-free 
melaphyres.  In  both  cases  there  has  been  a  great  addition  of  material  such  as  could 
not  have  been  brought  about  by  the  usual  agents  of  weathering. 

The  silicification  of  rhyolites  and  quartz-porphyries  or  their  tuffs,  and  the  develop- 
ment of  chalcedony  and  opal  in  all  kinds  of  igneous  rocks,  their  tuffs,  and  the  adjacent 
sediments,  belong  to  the  same  group  of  post-volcanic  processes.  Silicification  by 
weathering,  on  the  other  hand,  generally  produces  a  quartz-cement,  yet  such  cement 


POST-VOLCANIC  PROCESSES 


155 


may  be  also  abundantly  produced  by  post-volcanic  processes.  This  may  be  seen, 
for  example,  at  numerous  contacts  between  diabase  and  silicified  adinoles  and  lydites. 
The  formation  of  bauxite  is  of  a  different  character,  for  here  great  amounts  of  silica 
are  removed,  an  alteration  also  observed  in  some  basic  igneous  rocks. 

8.  Metasomatic  Replacement  of  Carbonate-rocks. — The  constituents  of  carbonate- 
rocks,  especially  of  limestones,  are  readily  soluble  in  various  solutions,  and  from  these 
solutions  new  minerals  crystallize.  Thus  ores  or  minerals  which  have  originated 
through  replacement  are  very  widely  distributed  in  limestones,  and  are  spoken  of 


Siderite  Marble 

FIG.  94. — Irregular  form  of  a  siderite  patch  in  marble. 


Huttenberg,  Karnten. 


as  metasomatic  (Gr.  /zerd,  after,  o-w/ia,  body).  These  deposits  are  extremely  irregular 
in  shape  (Fig.  94),  and  may  be  rather  sharply  separated  from  the  unaltered  limestones 
or  may  be  connected  with  them  by  transition  zones. 

Numerous  deposits  of  siderite,  rhodochrosite,  and  magnesite,  which  are  very  abun- 
dant in  the  altered  limestones  of  the  contact-zone  of  the  central  Alpine  granite,  belong 
to  this  class.  The  analogous  silicate  skarn  deposits  with  magnetite,  manganese 
silicates  and  oxides,  or  zinc  oxide,  etc.,  were  already  mentioned  on  page  129.  Finally, 
sphalerite,  galena,  and  calamine  may  occur  under  similar  conditions,  as  do  also  the 
silica  deposits  in  greenstone-schists  and  amphibolites. 


IX.  REGIONAL  METAMORPHISM 

LITERATURE 

J.  ROTH:  "tlber  die  Lehre  vom  Regionalmetamorphismus  und  die  Enstehung  der 

kristallinischen  Schiefer."     Abhandl.  preuss.  Akad.  Wiss.,  1871,  151.      Summar- 
izes the  older  literature. 

Idem:  "fitude  sur  les  schistes  cristallins."     IV  congr.  geol.  intern.,  Londres,  1888. 
F.A.  ADAMS  AND  J.  T.  NICOLSON:  "An  Experimental  Investigation  into  the  Flow  of 

Marble."    Phil.  Trans.  Roy.  Soc.,  London,  CVC  (1901),  363. 
A.  BALTZER:  "Der  mechanische  Kontakt  von  Gneis  und  Kalk  im  Berner  Oberland." 

Beitr.  geol.  Karte  Schweiz,  XX  (1880). 
F.  BECKE:  "Beziehungen  zwischen  Dynamometamorphose  und  Molekularvolumen." 

Neues  Jahrb.,   1896,  II,  182. 
Idem:  "tlber  Mineralbestand  und  Struktur  der  kristallinischen  Schiefer."     Denkschr. 

Akad.  Wiss.  Wein,  CXXV  (1903). 
H.CREDNER:  "Uber  nordamerikanische  Schieferporphyroide."     Neues  Jahrb. ,  1870, 

970. 
Idem:  "tTber  die  Genesis  der  archaischen  Gneisformation."     Zeitschr.  deutsch.  geol. 

Ges.,  XLII  (1890),  602. 

U.  GRUBENMANN:  "Die  kristallinischen  Schiefer."     I,  2  Aufl.,  Berlin,  1910. 
C.   W.    GUMBEL:  " Geognostische   Beschreibung  des  ostbayrischen  Grenzgebirges." 

Gotha,  1868. 
A.  HEIM:  "Untersuchungen  iiber  den  Metamorphismus  der  Gebirgsbildung."     Basel, 

1878. 

E.  KALKOWSKY:  "  tlber  die   Erforschung  der  archaischen   Formationen."     Neues 
Jahrb.,   1880,  I,   1. 

J.  LEHMANN:  "Die  Entstehung  des  altkristallinen  Schiefergebirges."     Bonn,  1884. 

R.  LEPSIUS:  "Geologic  von  Attika,"  Berlin,  1893. 

W.  LOSSEN:  "Geognostische  Beschreibung  der  linksrheinischen  Fortsetzung  des 
Taunus."  Zeitschr.  deutsch.  geol.  Ges.,  XIX  (1867),  509. 

L.  MILCH:  "Beitrage  zur  Kenntnis  des  Verrucano."    Leipzig,  1892  and  1896. 

Idem:  "Die  heutigen  Ansichten  iiber  Wesen  und  Entstehung  der  kristallinen  Schie- 
fer." Geol.  Rundschau,  I  (1910),  Hf.  3. 

F.  PFAFF:  "Mechanismus  der  Gebirgsbildung."     Heidelberg,  1880. 

H.  H.  REUSCH:  " Die  fossilfiihrenden  kristallinischen  Schiefer  von  Bergen."  Leipzig, 
1883. 

H.  ROSENBUSCH:  "Zur  Auffassung  des  Grundgebirges."     Neues  Jahrb.,  1889,  II,  81. 

Idem:  "Zur  Auffassung  der  chemischen  Natur  des  Grundgebirges."  Tscherm.  min. 
petr.  Mitt.,  XII  (1891),  49. 

A.  SAUER:  "Das  alte  Grundgebirge  Deutschlands."  Comptes  rendus  IX  congr., 
geol.  intern.,  1903,  Wein,  1904,  587. 

J.  J.  SEDERHOLM:  "tlber  den  gegenwartigen  Stand  unserer  Kenntnisse  der  Kristallin- 
ischen Schiefer  von  Finnland."  Comptes  rendus  IX  congr.  geol.  intern.,  1903, 
Wein  1904,  609. 

W.  SPRING:  "Recherches  sur  les  propriete"s  que  possedent  les  corps  solides  de  se 
souder  par  1'action  de  la  pression."  Bull.  acad.  roy.  Belgique,  1880,  323. 

156 


REGIONAL  METAMORPHISM  157 

P.  Termier:  "Les  schistes  cristallins  des  Alpes  occidentales."     Comptes  rendus  IX 

congr.  geol.  intern.,  1903,  Wein,  1904,  571. 
E.  WEINSCHENK  :  "  M6moire  sur  le  dynamo-me"tamorphisme  et  la  piezocristallisation." 

Comptes  rendus  VIII  congr.  geol.  intern.,  1900,  Paris,  1901,  326. 
Idem:  "Beitrage  zur  Petrographie  der    Zentralalpen,  speziell  des  Grossvenediger- 

stockes.  III.    Die    kootaktmetamorphe    Schieferhiille."     Abhandl.    bayr.    Akad. 

Wiss.,  II  Kl.,  XXII  (1903),  II  Abt.,  263. 
Idem:  "tlber  Mineralbestand  und  Struktur  der  kristallinischen  Schiefer."     Ibidem, 

XXII  (1906),  III  Abt.,  727. 

Early  Ideas  Regarding  the  Crystalline  Schists. — Geologists 
long  ago  recognized  the  existence  of  a  fundamental  difference 
between  the  so-called  fossiliferous  rocks  and  certain  unfossiliferous 
crystalline  schists  which  combine  the  characters  of  bedded  and 
crystalline  rocks.  Many  of  the  latter  were  definitely  known  to  be 
of  great  geologic  age,  therefore,  on  the  basis  of  their  petrographic 
characters,  the  generalization  was  made  that  all  schistose  crystal- 
line rocks  must  belong  to  the  same  primitive  Archean  formation, 
a  formation  supposed  to  consist  of  the  oldest  sediments  of  the 
earth. 

With  further  study  in  the  provinces  which  were  best  known  at 
that  time,  this  " primary"  schist  group  was  subdivided  into  gneiss, 
mica  schist,  and  phyllite.  Each  of  these  rocks  was  supposed  to  be 
as  characteristic  of  a  time-division  in  the  history  of  the  earth  as 
any  of  the  later  fossiliferous  formations.  Since  these  old  deposits 
were  unfossiliferous,  their  separation  is  necessarily  purely  petro- 
graphic and  depends  primarily  upon  the  fact  that  the  gneisses, 
which  are  the  very  oldest  formations,  are  chemically  allied  to 
granitic  rocks,  while  the  other  two  groups  are  analogous  to  later 
clastic  sediments.  Furthermore,  there  appears  to  be  a  gradual 
decrease  in  the  crystalline  character  of  the  rocks  from  the  gneisses 
to  the  phyllites,  and  most  of  the  upper  members  of  the  latter  group 
appear  to  be  transitional  to  clastic  rocks. 

The  assumption  that  the- Archean  represents  the  original  crust  of  the  earth  and 
the  first  chemical  precipitates  deposited  upon  it,  appears  at  first  sight  to  be  the  simp- 
lest and  most  natural  explanation,  and  the  fact  that  the  crystalline  schists  were  uni- 
versally present  wherever  deep  cuts  exposed  the  base  of  the  fossiliferous  formations, 
was  formerly  especially  emphasized.  Furthermore,  it  was  generally  found  that  the 
different  members  of  the  crystalline  schists  were  the  same  everywhere,  and  they 
occurred  in  the  same  sequence — gneiss,  mica-schist,  and  phyllite — as  they  neces- 
sarily should  if  they  represented  the  oldest  formations  of  the  earth's  crust. 

The  innumerable  inconsistencies  which  careful  study  reveals  between  the  actual 
relationships  and  these  theories  might  easily  be  overlooked  on  a  superficial  investiga- 
tion of  the  crystalline  schists.  Because  some  crystalline  schists  are  unquestionably 
pre-Cambrian,  all  have  been  assigned  to  this  age,  yet  in  by  far  the  most  localities 


158         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

they  are  overlaid,  not  by  the  oldest  fossiliferous  sediments,  but  by  very  much  younger 
ones,  into  which  the  schists  in  many  cases  pass  by  gradual  transitions.  On  the  other 
hand,  the  true  sediments  of  the  Cambrian,  and  those  of  the  pre-Cambrian  carrying 
occasional  fossils,  are  by  no  means  the  oldest,  non-crystalline  formations  of  the  earth, 
for  in  South  Africa  there  is  a  whole  series  of  non-fossiliferous  sediments  whose 
clastic  characteristics  are  hardly  altered,  yet  in  it  may  be  recognized  a  long  period  of 
pre-Cambrian  sedimentation,  divided  into  epochs  by  numerous  well-developed 
unconformities  and  transgressions. 

Younger  Crystalline  Schists. — Because  most  crystalline  schists 
contain  no  fossils,  they  were  thought  to  have  been  formed  before 
organic  life  was  possible  upon  the  earth,  hence  the  term  Azoic 
(Gr.  «,  without,  £coi>,  life)  was  applied.  Even  so  long  ago  as  the 
beginning  of  the  last  century,  occasional  fossils  had  been  found 
in  rocks  with  the  petrographic  character  of  normal  crystalline 
schists.  This  fact,  however,  did  not  disturb  the  belief  in  the  non- 
fossiliferous  character  of  the  old  crystalline  schist  formation,  for 
the  fossils  were  not  'especially  primitive  but  could  be  correlated 
with  certainty  with  type-fossils  of  younger  geologic  epochs.  Thus 
the  Silurian  graptolites  in  the  mica-schists  of  the  Bergen  peninsula 
in  Norway,  the  Jurassic  belemnites  in  those  of  the  St.  Gotthard 
region,  and  the  Carboniferous  plant  remains  in  the  phyllites  of 
the  Low  Tauern  mountains,  are  all  similar  to  the  type-fossils  and 
show  that  these  schists  unquestionably  belong  to  post-Archean 
formations.  Elsewhere,  gneisses  and  other  crystalline  schists  are 
found  interbedded  with  younger  formations,  for  example  in  the 
Cretaceous  in  Attica. 

These  " younger"  crystalline  rocks,  consequently,  were  differ- 
entiated from  the  so-called  Archean  schists  by  their  fossils,  and 
enormous  faults  were  artificially  established,  especially  in  the 
Alps,  to  separate  the  "true"  crystalline  schists  from  these  later 
imitators.  In  petrographic  character  these  younger  crystalline 
schists  correspond  so  completely  in  every  particular  with  those 
regarded  as  Archean,  that  from  this  standpoint  alone  they  must 
be  considered  identical.  By  so  regarding  them,  however,  the  whole 
basis  for  classifying  the  " crystalline  schists"  as  a  single  formation, 
namely  on  their  petrographic  character,  is  destroyed.  Here,  as 
everywhere,  the  fundamental  law  of  petrography  holds:  petro- 
graphic character  and  geologic  age  are  in  no  way  related. 

The  subject  cannot  be  dismissed  without  mentioning  the  fact  that  there  have  been 
found  locally  in  the  crystalline  schists,  certain  structures  which  seem  to  represent  or- 
ganic remains,  and  which  differ  from  all  known  fossils.  For  a  long  time  these  were 
regarded  as  examples  of  the  earliest  organisms  which  had  existed  upon  the  earth. 


REGIONAL  METAMORPHISM 


159 


One  of  these  apparent  fossils  is  Eozoon  Canadense  (Fig.  95).  This  consists  of  serpen- 
tine and  calcite  so  intergrown  that  it  externally  resembles  certain  organic  structures 
and  therefore  was  long  thought  to  be  a  variety  of  primeval,  giant  foraminifera,  coral, 
or  something  of  that  nature.  Careful  examinations  of  material  from  the  original 
locality,  Petite  Nation,  Canada,  and  of  all  similar  ophicalcites  from  Europe,  have 
clearly  shown,  however,  that  they  are  all  contact-metamorphosed,  forsterite-bearing 
limestones  and  not  of  organic  origin. 


FIG.  95. — Eozoon  Canadense.     Ophicalcite  from  Petite  Nation,  Canada. 

During  the  long  controversy  regarding  the  organic  nature  of  these  rocks,  a  note- 
worthy microscopic  feature  of  the  Canadian  occurrence  was  overlooked,  although  just 
this  feature  simulated  the  organic  structure,  namely,  a  worm-like  intergrowth  of  do- 
lomite and  calcite  (Fig.  96),  somewhat  resembling  pegmatitic  intergrowths  of  quartz 
and  feldspar.  Like  these,  it  probably  represents  a  eutectic  mixture.  Futhermore, 
absolute  proof  of  the  great  age  of  the  Laurentian  deposits  in  which  the  Eozoon  Canad- 
ense was  found  does  not  appear  to  have  been 
given,  the  age  assumption  resting  upon  the 
petrographic  character  of  the  rocks. 

It  is  noteworthy  that  all  the  known  fossils 
of  the  crystalline  schists,  so  far  as  their  ages 
have  been  determined  with  anything  like 
certainty,  belong  to  relatively  young  geologic 
formations.  An  examination  of  these  so- 
called  oldest  sediments  in  various  localities 
leads  to  the  conclusion  that  they  are  un- 
questionably not  the  oldest  fossil-bearing 
sediments.  The  same  conclusion  is  reached 
by  a  study  of  the  life  development  in  the 
Cambiran,  its  well-developed  fauna  showing 
that  it  must  have  been  preceded  by  a  long 
series  of  more  primitive  forms.  Nowhere  in 
the  crystalline  schists,  however,  have  fossils 
bearing  the  characteristics  of  primeval  forms 
been  found.  This  has  been  explained  as 
being  due  to  the  lack  of  resistant  hard  parts  in  these  primitive  forms,  an  explanation 
also  given  for  the  fossil-free  character  of  the  extensive  pre-Cambrian  formations  of 
South  Africa. 

If,  however,  conclusions  derived  from  historical  geology  are  justifiable,  there  can 
be  no  doubt  that  Cambrian  fossils  must  have  had  a  long  series  of  ancestors  with  pre- 
servable  skeletons.     That  they  have  not  been  found  has  always  been  a  stumbling- 
11 


FIG.  96. — Eutectic  intergrowth  of 
calcite  and  dolomite  in  Eozoon.  Petite 
Nation,  Canada. 


160         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

block  to  geologists,  and  it  seems  impossible  to  explain  their  disappearance  except  on 
the  assumption  of  great  disturbances  of  the  earth's  crust  at  the  beginning  of  the 
Cambrian,  perhaps  corresponding  to  the  Catastrophic  Period  of  Stiibel. 

Variability  of  the  Crystalline  Schists. — The  great  variability 
in  the  composition  of  the  different  members  of  the  crystalline 
schists  series  has  already  been  pointed  out.  It  is  so  pronounced 
that  it  can  be  overlooked  only  on  the  most  superficial  examination. 
It  is  true  that  the  rocks  of  one  formation  are  embraced  under  the 
name  of  gneiss,  but  the  name  is  more  geologic  than  petrographic, 
and  if  the  rocks  are  examined  carefully,  all  ideas  of  even  approxi- 
mately similar  characters  must  be  dismissed  at  once. 

The  individual  members  of  the  gneiss  series  have  only  two 
properties  in  common:  (1)  they  have  a  more  or  less  clearly  recog- 
nizable parallel  structure,  and  (2)  they  usually  contain  the  minerals 
quartz,  feldspar,  and  mica.  The  gneisses  of  the  Alps,  the  Bavarian 
Forest,  the  Erzgebirge  of  Saxony,  etc.  are  so  very  different  in 
texture,  in  the  occurrence  of  accessory  minerals,  and  in  the  relative 
proportions  of  the  chief  constituents,  that  geologists  accustomed 
to  work  in  one  region  are  hardly  willing  to  regard  the  rocks  in 
another  as  equivalent  formations. 

Furthermore,  innumerable  rocks,  granular  limestones,  gabbros, 
eclogites,  amphibolites,  serpentines,  or  ores,  are  interbedded  with 
the  gneiss,  either  in  schistose  or  lens-like  masses,  or  forming  an 
entire  horizon,  as  granulite  does  in  the  Saxon  Granulitgebirge,  in 
Bohemia,  and  elsewhere.  In  short,  the  whole  appearance  of  the 
formation  differs  from  that  which  one  naturally  would  have 
expected  of  the  first  solid  crust  of  the  earth. 

The  conditions  in  the  mica-schist  formation  are  still  more 
confused.  Besides  the  mica-schists  proper,  all  of  the  characteristic 
rocks  of  the  gneiss  formation,  amphibolites,  chlorite-schists,  green- 
stone-schists, calcareous-mica-schists,  quartzites,  etc.,  again  appear. 
The  same  variability,  though  to  a  lesser  degree,  is  characteristic 
for  the  phyllites,  the  upper  group  of  the  crystalline  schist  formation. 

A  thorough  geologic  and  petrographic  study  of  the  crystalline 
schists  shows,  without  question,  that  these  rocks  by  no  means 
represent  a  universal  formation.  The  only  characteristic  common 
to  all  of  them  is  their  variability  and  not  their  uniformity,  although 
the  latter  might  be  presupposed  as  an  essential  of  the  earliest 
crust  of  the  earth.  It  follows,  therefore,  that  the  crystalline  schists 
cannot  be  portions  of  this  primeval  crust. 


REGIONAL  METAMORPHISM  161 

GiimbePs  Theory  of  Diagenesis. — Gumbel  attempted  in  a 
measure  to  overcome  the  difficulties  brought  out  by  the  characters 
of  these  rocks  by  means  of  his  theory  of  diagenesis  (Gr.  Sid  ,through, 
yljvofjiai,  to  be  born).  According  to  this  theory,  the  crystalline 
schists,  which  were  originally  normal,  clastic  sediments,  have 
undergone  much  greater  alteration  than  the  sediments  of  later 
geologic  formations.  This  is  due  primarily  to  the  fact  that  the 
water  of  the  earliest  periods,  on  account  of  its  higher  temperature 
and  greater  content  of  chemically  active  agents,  had  a  much 
greater  power  to  dissolve  and  rebuild  than  it  had  later.  The 
recrystallization  of  the  ooze,  thoroughly  saturated  as  it  was  with 
hot  sea  water,  to  gneiss  or  mica-schist  is  hard  to  understand, 
especially  when  the  unimportance  of  diagenesis  in  later  formations 
is  considered.  And  furthermore,  only  the  insoluble  residues  were 
left  to  be  re-formed  by  diagenesis,  the  primeval  ocean  having 
undoubtedly  leached  and  precipitated  elsewhere  some  of  the 
materials  from  the  particles  of  ooze  still  floating  in  it. 

The  theory  of  diagenesis  depends  upon  the  assumptions  that 
all  of  the  formations  embraced  in  the  Archean  group  represent 
one  definite  time  period,  that  they  were  deposited  before  the  oldest 
fossiliferous  strata,  and  that  during  this  time  the  physical  condi- 
tions at  the  surface  of  the  earth  were  not  as  yet  suitable  for  life, 
or,  if  they  were  so,  only  for  very  low  forms,  These  assumptions 
are  accepted  as  a  matter  of  course  by  most  geologists,  although  the 
existence' of  crystalline  schists  of  definitely-known  younger  age 
alone  should  serve  as  a  warning  that  care  must  be  taken  in 
using  petrographic  characteristics  as  indicative  of  definite  age 
relationships. 

The  crystalline  schists  of  the  Pyrenees  which  are  overlaid  by  Upper  Silurian  beds, 
and  the  formations  of  the  Central  Alps  overlaid  by  Triassic  and  Jurassic,  cannot  be 
called  pre-Cambrian  simply  because  they  correspond  petrographically  with  rocks 
which  are  known  to  be  pre-Cambrian. 

It  is  hardly  conceivable  that  normal  Cambrian  or  Silurian  sediments  could  be 
deposited  in  one  place  on  the  earth's  surface  while  at  the  same  time  crystalline  rocks 
were  directly  formed  in  another.  The  conditions  necessary  for  the  development  of 
primary  crystalline  schists  are  so  different  from  those  of  normal  sedimentation,  that 
the  simultaneous  formation  of  the  two  seems  impossible.  One  must  assume,  therefore, 
as  geologists  generally  do,  that  the  two  groups  are  of  different  ages,  and  all  the  crystal- 
line schists  are  Archean  or  pre-Cambrian;  or  that  the  crystalline  schists  were  originally 
true  sediments  which  developed  a  crystalline  texture  subsequent  to  their  deposition, 
by  some  process  independent  of  sedimentation.  The  "younger"  crystalline  schists 
definitely  show  that  such  metamorphism  may  take  place,  and  their  similarity 
to  true  Archean  rocks  suggested  the  possibility  that  these  older  crystalline  schists 


162         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

were  produced  by  some  widespread  alteration  process.     In  this  way  the  theory  of 
general  or  regional  metamorphism  originated. 

Theories  of  Regional  Metamorphism. — Regional  metamorphism, 
as  opposed  to  contact-metamorphism  which  is  distinctly  a  local 
phenomenon,  has  been  regarded  as  due  to  alteration  processes 
which  have  a  very  extensive  zone  of  action.  All  theories  of 
regional  metamorphism  proceed  from  the  conviction  that  the 
crystalline  schists  generally  do  not  possess  the  characteristics  of 
primary  formations.  If,  therefore,  these  rocks  did  not  originate 
from  a  primitive  crust  and  the  oldest  chemical  sediments,  they 
undoubtedly  correspond  to  sediments  and  igneous  rocks  which 


FIG.  97. — Injection  schist.     Freiberg,  Saxony. 

occur  in  an  unaltered  condition  on  other  parts  of  the  earth's  sur- 
face. The  chemical  composition  of  these  altered  rocks  indicates 
the  character  of  the  original  material ;  simple  recrystallization  does 
not  produce  great  chemical  alterations,  and  there  is  a  character- 
istic chemical  difference  between  clastic  and  igneous  rocks,  as 
was  shown  in  Sections  V  and  VI. 

The  crystalline  schists  have  been  classified,  for  example  by  Rosenbusch,  on  the 
basis  of  their  chemical  compositions.  To  such  a  classification  the  objection  has  been 
made  that,  owing  to  the  great  variation  in  the  character  of  sediments,  an  arkose  or 
greywacke  may  somewhere  occur  which  corresponds  closely  to  granite  in  chemical 
composition.  While  this  is  possible  it  is  very  improbable,  and  the  chances  are  very 
slight  that  a  gneiss  of  granitic  composition  was  originally  such  a  greywacke.  On 
the  other  hand,  the  igneous  rocks  may  have  undergone  extensive  alteration  before 
metamorphism,  and  thus  a  further  disturbing  factor  is  introduced.  Such  alteration 


REGIONAL  METAMORPHISM  163 

doubtless  occurred  in  many  cases,  but  the  results  of  normal  weathering  would  have 
slight  influence  on  the  enormous  rock-complexes  from  which  the  crystalline  schists 
must  have  been  formed.  With  rare  exceptions,  the  chemical  compositions  of  the  re- 
placement products,  which  were  discussed  in  Section  VIII,  differ  just  as  much  from 
the  chemical  composition  of  the  sediments,  as  do  those  of  the  original  igneous  rock 
itself. 

The  chemical  composition  of  a  crystalline  schist  is  always  an  important  aid  in 
the  determination  of  the  character  of  the  original  rock,  and  in  certain  cases  it  alone 
is  sufficient.  In  other  cases  it  may  be  necessary  to  confirm  the  determination  by 
means  of  the  geologic  mode  of  occurrence.  If  the  gneiss  formation,  for  example,  is  exam- 
ined by  these  methods,  it  will  be  found  that  two  end-members  are  easily  distinguished. 
The  lower  part  of  the  series,  as  a  rule,  shows  the  characteristics  of  true  granites, 
quartz-diorites,  etc.,  while  the  upper  corresponds  to  greywackes,  slates,  and  the  like. 


FIG.  98. — Granite  made  schistose  by  resorption.     Mulda  near  Freiberg,   Saxony. 

The  latter,  also,  is  distinguished  from  the  former  by  its  much  more  abrupt  variation 
in  habit  and  composition. 

According  to  their  mode  of  origin,  these  rocks  are  called  igneous  gneisses  or  sedi- 
mentary gneisses,  or,  following  Rosenbusch,  orthogneisses  or  paragneisses.  Between 
the  two,  however,  there  is  a  third  equally  justifiable  group  which  possesses  the  chem- 
ical characteristics  of  neither  igneous  nor  sedimentary  rocks.  Rosenbusch  classified 
them  as  metagneisses.  Certain  of  these  rocks  usually  show  rather  perfect  schistosity 
and  banding  and,  like  the  paragneisses,  vary  abruptly  in  composition.  These  rocks 
are  in  part  schists  injected  with  granitic  magma  (Fig.  97),  in  which  the  phenomenon 
of  injection  can  be  still  distinctly  recognized  megascopically  and  microscopically, 
in  part  mixed  rocks  in  which  the  exfoliated  schist  was  more  or  less  completely  resorbed 
by  the  granitic  magma  (Fig.  98). 

In  many  cases  the  conclusions  as  to  the  original  character  of  a  crystalline  schist, 
drawn  from  its  chemical  analysis,  may  be  confirmed  by  preserved  remnants  of  the 
original  texture.  Thus,  large  quartz  boulders,  almost  unaltered,  may  be  preserved  in 
metamorphosed  rocks,  or  fragments  of  fossils  may  be  found.  In  such  cases  there  can  be 
no  question  as  to  the  sedimentary  origin  of  the  rock.  On  the  other  hand,  the  gneisses 


164         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

of  the  central  Alps  show,  besides  the  usual  granitic  texture,  all  the  other  external 
characters  of  intrusive  rocks,  such  as  inclusions  of  the  country-rock,  apophyses,  etc. 
Within  them,  also,  dikes  of  chlorite-schist  occur,  and  they  may  show  more  or  less  sharply 
defined  light  spots  with  the  outlines  of  the  phenocrysts  of  the  original  porphyrite 
(Cf.  Fig.  83).  Further,  in  the  uralitic  hornblende  of  the  amphibolites  there  may  still 
be  distinct  traces  of  the  parallel  diallage  inclusions,  which  are  so  characteristic  of 
gabbro.  Thus  similar  examples  could  be  presented  for  almost  all  varieties  of 
crystalline  schists,  definitely  proving  that  these  so-called  primitive  rocks  embraced 
both  igneous  rocks  and  sediments. 

An  objection,  which  applies  equally  well  to  all  theories  of  regional  metamorphism, 
is  that  even  in  the  very  oldest  sediments,  namely,  the  basal  conglomerates  of  the  oldest 
fossiliferous  formations,  there  have  been  found  occasional  boulders  petrographically 
resembling  crystalline  schists.  According  to  the  theories  of  metamorphism,  if  rounded 
fragments  of  mica-schist  and  phyllites  occur  in  the  overlying  Cambrian  strata,  these 
boulders  must  have  been  metamorphosed  before  the  Cambrian  rocks  were  deposited. 
In  other  words,  the  metamorphism  must  have  taken  place  after  the  deposition  of  the 
upper  beds  of  the  phyllite  and  before  the  deposition  of  the  lower  beds  of  the  Cambrian, 
in  spite  of  the  fact  that  the  two  series  are  connected  in  many  cases  by  all  possible 
transitions.  Such  occurrences  will  be  discussed  in  greater  detail  at  the  conclusion  of 
this  section. 

Three  theories  of  regional  metamorphism  are  of  especial 
importance:' 

1.  Plutonic  (Pluto,  God  of  the  underworld)  or  anogenic  (a  va, 
up,  ylyvofjiai,  to  be  born)  metamorphism. 

2.  Hydrochemical  (Gr.  #5 up,  water),  Neptunian  (Neptune,  God 
of  the  sea),  or  katogenic  (Gr.  K<XTCO,  down)  metamorphism. 

3.  Dislocation  (Lat.  dislocare,  to  displace)  or  dynamometamor- 
phism  (Gr.  dvvafjus,  force). 

Plutonic  and  Hydrochemical  Metamorphism. — The  theory  of 
plutonic  metamorphism,  which  was  proposed  by  Hutton  and 
geologically  applied  by  Ami  Boue  and  Lyell,  is  based  upon  the 
temperature  gradient  of  the  earth,  the  heat  of  the  interior  (or, 
in  the  language  of  Hutton,  "the  subterranean  fire")  being  re- 
sponsible for  the  alteration.  If  sedimentation,  in  some  locality, 
increases  the  thickness  of  the  earth's  crust  by  thousands  of  meters, 
it  causes  the  lowest  beds  to  pass  gradually  into  the  zone  of  fusion. 
Here,  with  the  aid  of  moisture,  recrystallization  takes  place,  and 
the  originally  clastic  rock  is  metamorphosed  into  one  which  is 
crystalline.  More  recently  Termier  substituted  "mineralizers," 
for  "moisture"  in  this  theory;  the  mineralizers  being  supposed  to 
pass  from  the  molten  magma  of  the  interior  into  the  overlying 
sediments  (cf.  page  65).  Since  the  metamorphic  agents  act  from 
below,  the  lowest  beds  are  most  completely  changed.  Recent 
formations  are  altered  in  fewer  cases  than  are  the  older,  since  they 


REGIONAL  METAMORPHISM  165 

are  more  rarely  covered  by  deep  sediments.     Finally  the  altered 
formations  again  appear  on  the  surface  by  denudation. 

Since  the  temperature  gradient  is  only  about  30°  per  kilometer  and  probably 
decreases  with  depth,  this  theory  appears  absolutely  untenable.  To  produce  the 
required  alteration  there  must  have  been  an  increase  of  many  hundred  degrees.  This 
would  have  necessitated  the  deposition  and  subsequent  removal  of  sediments  many 
kilometers  in  depth  wherever  crystalline  schists  appear  on  the  surface  of  the  earth. 

Contrary  to  this  hypothesis  is  that  of  hydrochemical  meta- 
morphism,  which  was  placed  on  a  scientific  basis  primarily  by 
Bischof.  In  this  theory,  vadose  waters  are  considered  the  active 
agents.  They  force  themselves  into  the  rocks,  and  becoming 
saturated  with  various  substances,  produce  alterations  in  the 
different  formations.  The  process  is  the  reverse  of  weathering, 
for  instead  of  producing  disaggregation  of  the  rocks  there  is  an 
inner  recrystallization,  in  the  place  of  the  development  of  new 
hydrous  products  there  is  a  withdrawal  of  water,  and  instead  of 
the  leaching  of  constituents  there  is  an  addition  of  material. 
Hydrochemical  metamorphism  never  became  more  than  a  hypoth- 
esis. Not  only  was  it  unsupported  by  observations,  but  the 
phenomena  of  weathering  under  all  conditions  are  opposed  to  it. 

Latent  Plasticity  and  Fractureless  Folding. — Since  the  theory 
of  dynamometamorphism  is  now  the  one  most  generally  accepted 
by  petrographers  as  well  as  by  geologists,  it  may  be  well  to  enquire 
into  its  justification.  The  theory  was  proposed  by  Lossen,  and 
was  developed  from  the  petrographic  side  by  Rosenbusch  and  his 
pupils,  from  the  geologic  side  by  Heim  and  Baltzer,  and  from  the 
physico-chemical  side  by  Becke. 

In  the  process  of  mountain-building  the  rocks  undergo  many 
alterations,  some  of  the  changes  being  megascopically  visible  in 
nearly  every  disturbed  region.  Sediments  primarily  are  influenced 
by  these  erogenic  processes.  The  beds  are  greatly  folded  (Fig.  99), 
relatively  recent  rocks  may  become  as  compact  as  older  formations, 
and  where  there  is  transverse  cleavage  it  is  not  uncommon  to  find 
mica-like  minerals  upon  the  joint-planes.  Under  the  influence  of 
great  stresses  the  rocks  no  longer  act  as  rigid  bodies  but  acquire 
a  certain  degree  of  plasticity.  This  enables  them,  within  certain 
limits,  to  yield  to  the  pressure  and  change  their  form.  Heim  calls 
this  phenomenon  "  f ractureless  folding,"  and  says  it  occurs  wher- 
ever rocks  are  subjected  to  great  and  gradual  lateral  stresses.  The 
vertical  load  prevents  movement  in  that  direction,  a  movement 


166         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

which  would  otherwise  rapidly  relieve  the  strain.  Heim  supposed 
that  under  the  enormous  load  of  the  overlying  beds,  the  rocks 
reached  a  potentially  plastic  condition.  This  permitted  the 
molecules  to  equalize  the  differential  stresses  by  gliding  and  by 
displacements,  with  the  result  that  the  rocks  were  able  to  adjust 
themselves  to  the  new  conditions  without  making  necessary  an 
internal  fracturing  of  the  individual  components. 

The  theory  of  fractureless  folding,  which  was  based  upon 
superficial  megascopic  observations,  was  long  ago  shown  by  micro- 
scopic study  to  be  of  very  limited  application.  It  is  true  that  a 


FIG.  99. — Folded    Devonian    limestone.     Barrandefelsen,    Bohemia.     (H.    Eckert, 

Prag,  Photo.) 

certain  degree  of  plasticity  is  the  property  of  all  rocks,  and  that  it 
is  developed  best  where  a  slowly-moving  force  acts  upon  them 
under  an  especially  great  load.  The  range  of  the  elastic  limits  of 
most  rocks,  however,  is  rather  narrow,  and  rocks  which  mega- 
scopically  appear  to  be  folded  without  fractures  are  usually  shown 
by  the  microscope  to  consist  of  mutually  displaced  fragments  of 
the  various  constituents,  or  of  internally  fractured  and  re-cemented 
minerals. 

Many    veined    gneisses    show    a    remarkable    folding.     For 
example,  the  light  bands  in  Fig.  100  are  granite-aplite,  andUts 


REGIONAL  METAMORPHISM  167 

constituents,  especially  quartz,  are  extremely  brittle,  yet  they 
show  no  trace  of  fracturing  even  on  the  most  thorough  microscopic 
examination.  It  was  therefore  thought  that  these  rocks  were 
examples  of  perfect  fractureless  folding.  A  much  more  probable 
explanation  of  such  rocks  is  that  the  aplite  penetrated  the  loosened 
joints  of  a  previously  crumpled  rock.  After  the  solidification  of 
the  aplite  and  the  alteration  of  the  sediment  by  contact-meta- 
morphism,  no  folding  took  place. 

The  degree  to  which  a  rock  may  be  folded  without  fracturing  depends  primarily 
upon  its  composition  and  texture.  Slates,  and  in  fact  all  rocks  which  were  built  up 
predominantly  of  colloids,  loose  aggregates  of  readily  displaceable  minerals,  or  of 
flakes  of  mica-like  minerals  which  may  be  readily  bent,  are  most  plastic.  Lime- 
stone, under  great  pressure,  may  likewise  undergo  considerable  re-formation  without 


FIG.  100. — Banded  gneiss.     Untersulzbachtal,  Grossvenediger.     The  contorted  aplite 
veins  show  sharp  contacts  against  the  schist. 

breaking.  This  is  probably  due  to  the  ability  of  calcite  to  form  twins  (Fig.  4,  PL  VI). 
Dolomite,  however,  does  not  possess  the  property  of  gliding,  and  even  under  great 
pressure,  mashing  only  leads  to  a  general  fragmentation  or  endogenic  brecciation. 
This  is  brought  out  clearly  by  weathering,  for  such  rocks  disintegrate  into  a  mass  of 
minute,  angular  fragments  of  dolomite,  the  calcareous  cement  having  been  dissolved 
out  by  the  atmospheric  agents.  The  plasticities  of  feldspar,  quartz,  and  olivine  are 
very  slight.  The  latter  two  are  among  the  most  brittle  of  the  rock-forming  minerals, 
for  they  break  upon  the  slightest  orogenic  movement.  Thus  the  degree  of  internal 
fracturing,  shown  by  the  cataclastic  texture,  serves  as  a  measure  of  the  pressure. 

Rocks  composed  of  several  minerals  may  act  quite  differently  under  pressure, 
owing  to  their  textural  differences.  Thus,  a  sandstone,  composed  of  quartz  grains 
surrounded  by  a  clay-like  cement,  may  be  considerably  folded  without  being  fractured 
internally,  while  the  constituents  of  a  granite,  under  the  same  conditions,  would  be 
completely  mashed.  A  very  plastic  rock,  interbedded  with  one  which  consists  pre- 
dominantly of  brittle  minerals,  will  be  folded  without  breaking;  the  enclosing  brittle 
rock,  however,  will  be  crushed,  and  into  its  open  fractures  the  plastic  mineral  of  the 
former  rock  will  be  forced.  An  example  of  this  is  the  diabase  dike  in  Fig.  101.  This 


168 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


has  been  broken  up  and  now  occurs  in  disconnected  lens-like  masses  separated  by 
the  plastic  enclosing  limestone.  Similarly,  where  limestones  and  brittle  rocks  are 
folded  together,  the  former  are  forced  in  dike-like,  pseudo-igneous  forms  across  the 
schistosity  and  into  fissures  radiating  from  the  crests  and  troughs  of  the  folds  of  the 
brittle  rock. 

The  plasticity  of  many  minerals  is  considerably  increased  by  heat,  especially 


FIG.  101. — Broken  diabase  dike  in  marble.     Wunsiedel,  Fichtelgebirge. 


when  accompanied  by  gases  and  vapors,  even  though  the  temperature  remains  below 
that  at  which  they  soften.  This  has  been  shown  in  a  remarkable  manner  by  experi- 
ments on  limestones  and  marbles.  The  fractureless  deformation  of  calcite  grains  in 
contact-metamorphosed  rocks  is  thus  accounted  for. 

Such  a  fractureless  bending,  however,  cannot  be  produced  experimentally  in  the 
feldspars,  quartz,  or  other  important  rock-forming  minerals.     Their  elastic  limits  are 

reached  as  quickly  under  the  conditions 
of  contact-metamorphism  as  under  dyna- 
mometamorphism,  and  instead  of  being 
folded  without  fractures,  they  are  intern- 
ally shattered.  A  granite,  for  example, 
even  under  the  most  favorable  conditions, 
shows  no  evidence  of  having  become 
plastic,  and  there  is  no  adjustment  of 
its  borders  to  that  of  the  country-rock, 
nor  is  it  ever  forced,  in  the  solidified 
state,  into  fissures  or  joints  of  the  coun- 
try-rock. This  artificial  hypotheses  of 
plasticity  has  resulted  in  the  misinter- 
pretation of  normal  phenomena  of  ig- 
neous sheets  in  every  case  where  the  field 
distortion  of  granite  or  gneiss  has  been 
explained  as  due  to  a  softening  of  the 
rock. 

An  alteration  in  form  without  com- 
plete fracturing  is  no  more  possible  for 
the  quartz  phenocrysts  in  certain  quartz-porphyries  and  the  quartz  grains  in  certain 
sandstones,  even  under  the  action  of  pressure  or  tension,  than  it  is  for  the  same  min- 
erals in  thousands  of  other  cases  where  they  have  shown  themselves  to  be  brittle,  and 
not  plastically  deformable.  The  stretched  quartzes  in  the  quartz-porphyry  of  Thale 
in  Thiiringen  (Fig.  102),  called  Kaulquappenquarze  (tad-pole  quartz)  from  their 
form,  and  the  elongated  quartz  grains  in  certain  "stretched"  sandstones,  both  of  them 
explained  as  due  to  plastic  deformation,  are  doubtless  simply  primary  forms  in  the 


FIG.  102. — Tad-pole-quartz-porphyry. 
Thale,  Thiiringen.  (Prof.  Dr.  Klemm, 
photo.) 


REGIONAL  METAMORPHISM  169 

case  of  the  porphyry,  and  recrystallizations  by  contact-metamorphism  under  strong 
lateral  pressure  in  the  sandstones. 

Dynamometamorphism. — Helm's  view  that  rocks  possess  latent 
plasticity  when  under  the  weight  of  overlying  strata,  is  modified 
by  believers  in  the  theory  of  dynamometamorphism  only  by  the 
assumption  that  the  overlying  load  produces  in  the  solid  rock  a 
certain  mobility  of  molecules  which  permits  them  to  react  upon 
each  other  and  finally  leads  to  a  complete  molecular  re-arrange- 
ment. Whether  moisture  or  the  heat  set  free  by  mechanical 
re-adjustments  aids  these  chemical  reactions  is  immaterial,  the 
main  factor  in  producing  the  mobility  is  undoubtedly  pressure. 

The  theory  of  dynamometamorphism  is  based  upon  the  facts 
that  the  crystalline  schists,  in  many  cases,  show  the  effects  of 
exceptionally  great  earth-movements,  and  that  the  crystallinity 
of  a  sediment  or  the  schistosity  of  an  igneous  rock  increases  more 
and  more  with  the  amount  of  disturbance.  In  innumerable  cases 
the  amount  of  compression  has  been  estimated  solely  from  the 
degree  of  crystalline  schistosity  of  the  rock  itself;  the  molecular 
re-arrangement,  supposed  to  originate  in  pressure,  being  at  the 
same  time  the  only  evidence  that  such  pressure  was  especially 
active.  The  theory,  therefore,  is  presupposed  to  be  correct  to 
furnish  evidence  that  it  is  correct. 

The  experimental  work  of  Spring  is  generally  thought  to  support  the  theory  of 
dynamometamorphism.  He  found  that  a  dry  mixture  of  flowers  of  sulphur  and  cop- 
per shavings  could  be  transformed,  under  a  pressure  of  several  thousand  atmospheres 
and  without  increasing  the  temperature,  to  black  crystalline  chalcocite.  The  funda- 
mental basis  of  dynamometamorphism  is  the  possibility  of  a  molecular  re-arrange- 
ment in  the  solid  state.  That  this  occurs  must  be  granted,  not  only  from  Spring's 
experiments  but  from  later  ones  made  with  various  substances,  especially  with  metallic 
salts.  Spring's  experiment,  however,  loses  much  of  its  convincing  power  when  the 
results  obtained  with  rock-making  materials  are  examined,  for  under  pressure  as  great 
as  20,000  atmospheres,  pulverized  lime-carbonate,  silicic  acid,  etc.,  presented  no  traces 
of  alteration.  Experiments  show,  therefore,  that  while  the  mobility  of  the  molecules 
of  certain  substances  is  great  enough  to  permit  recrystallization  under  the  stresses 
which  may  exist  during  orogenic  movements,  the  degree  of  this  mobility  varies  greatly. 
In  the  minerals  forming  the  crystalline  schists  especially,  it  is  so  slight  that  it  seems 
hardly  sufficient  to  account  for  a  molecular  re-arrangement  of  the  rock,  even  when 
enormous  loads  and  long  periods  of  time  are  assumed. 

The  results  of  Spring's  experiment  are  those  to  be  expected  theoretically;  many 
of  the  metals,  metallic  sulphides,  etc.,  are  of  less  volume  when  fluid  than  when  solid; 
the  silicates,  on  the  other  hand,  expand  on  melting.  In  the  first  case  an  increase  in 
pressure  must  produce  a  lowering  of  the  melting  point,  in  the  second  the  reverse  is  true. 

Experiments  have  recently  been  made  in  regard  to  the  plasticity  of  minerals  and 
rocks,  more  especially  of  calcite  and  marble.  The  results  show  that  while  great 
pressure  can  alter  the  form  of  the  minerals,  it  cannot  do  so  without  at  the  same  time 


170         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

completely  mashing  them.  It  is  true  that  these  masses  develop  a  certain  coherence, 
but  it  is  only  such  as  any  powder  will  take  on  when  greatly  compressed.  Recrystal- 
lization  only  occurs  under  this  great  pressure  when  at  the  same  time  the  thoroughly 
moistened  material  is  heated  several  hundered  degrees. 

Riecke  undertook  certain  experiments  to  determine  the  cause 
for  the  molecular  re-arrangement  and  the  extensive  schistosity  of 
metamorphosed  rocks.  He  found,  in  a  saturated  solution,  that 
pressure  increased  the  solubility  of  a  crystal  in  the  direction  of  the 
pressure,  and  decreased  it  at  right  angles  to  this  direction.  Riecke' s 
principle  was  applied  by  Becke  to  the  dynamometamorphic  altera- 
tion of  crystalline  schists.  He  regarded  the  ground-water,  which 
circulates  in  all  the  pores  of  the  rocks,  as  a  concentrated  solution 
of  all  the  rock-constituents.  Lateral  orogenic  pressure,  decreasing 
the  solubility  of  the  constituents  in  the  direction  at  right  angles  to 
it,  caused  them  to  grow  in  this  direction,  which  usually  also  corre- 
sponds to  that  of  the  schistosity  of  the  rocks.  The  circulating 
water  continually  brings  in  new  material  for  this  growth  by  dis- 
solving the  constituents  on  the  sides  from  which  the  pressure 
comes. 

The  importance  of  the  ground-water  as  a  factor  in  the  alteration  of  rocks  dimin- 
ishes rapidly  with  the  depth.  The  decrease  in  the  water  content  of  rocks  on  account 
of  the  overlying  load,  and  the  dust-dry  condition  of  the  walls  in  fairly  deep  mines, 
makes  it  seem  probable  that  water  exists  only  in  the  theory.  Even  if  present,  the 
ground-water  can  be  of  only  slight  importance  as  a  solvent  for  the  difficultly  soluble 
constituents  of  the  silicate  rocks.  Assuming  that  the  solubility  of  these  constituents 
does  increase  with  an  increase  in  temperature,  it  is  hardly  probable,  on  account  of 
the  slowness  of  the  mountain-forming  movements  and  in  spite  of  the  poor  heat- 
conductivity  of  rocks,  that  there  would  be  enough  heat  generated  by  the  friction  of 
the  particles  during  the  re-formation  of  the  rocks  to  cause  solution. 

The  development  of  considerable  heat  over  wide  areas,  even  though  the  mountain- 
folding  took  place  relatively  rapidly,  is  purely  hypothetical.  The  fact  that  the  frag- 
mental  material  occurring  in  regions  characterized  by  the  greatest  faulting  and  even 
thrust  faulting,  shows  no  suggestion  of  a  temperature  increase  such  as  would  lead  to 
molecular  re-arrangements,  is  strong  evidence  against  this  theory.  The  crushed 
condition  of  the  rocks  and  the  magnitude  of  many  of  the  faults  show  that  there  must 
have  been  great  friction,  consequently  local  molecular  re-arrangements  might  well 
be  expected,  but  are  nowhere  found,  in  the  normal  friction-breccias,  or  so-called 
mylonites. 

Further,  in  numerous  cases  sediments  have  been  squeezed  and  intensely  folded 
without  showing  a  trace  of  crystalline  alteration.  The  Tertiary  deposits  of  the  Glar- 
ner  Alps,  for  example,  are  deformed  to  a  degree  rarely  found  even  in  the  crystalline 
schists,  but  while  they  possess  transverse  schistosity,  they  show  no  trace  of  crystal- 
linity.  The  Lochseite  limestones  also  show  the  effects  of  very  great  mechanical 
movements,  but  while  they  have  been  actually  kneaded  together  (Fig.  103),  their 
character  is  absolutely  different  from  that  of  the  limestones  of  the  crystalline  schists. 
No  trace  of  newly  formed  minerals  or  of  recrystallizations  due  to  great  pressure 
appears,  not  even  microscopically. 


REGIONAL  METAMORPHISM 


171 


On  the  other  hand,  the  Carboniferous  phyllites  of  the  Low  Tauern  contain  frag- 
ments of  rocks  which  show  no  traces  of  mechanical  strains.  They  contain  plant  re- 
mains which  show  no  signs  of  corrosion  and  graphite  which  still  preserves  the  struc- 
ture of  the  coal  from  which  it  was  derived,  yet  the  schists  themselves  are  entirely 
crystalline  in  texture.  Further,  in  many  places  in  the  Alps,  as  in  the  Stubai  group, 
Monte  Rosa,  etc.,  highly  crystalline,  altered  sediments  occur  in  practically  undis- 
turbed and  nearly  horizontal  beds.  The 
same  is  true  of  the  recent,  crystalline 
schists  of  Attica.  The  general  term  dyna- 
mometamorphism,  therefore,  does  not 
correctly  indicate  the  actual  phenomena. 

The  theory  of  dynamometa- 
morphism  was  chiefly  developed 
in  the  western  Alps,  where  it 
was  based,  primarily,  upon  as- 
sumptions which  are  demon- 
strably  false.  The  gneiss-like 
character  of  the  central  Alpine 
granites  misled  geologists,  and 
caused  them  to  classify  the  rocks 
as  gneisses.  Further,  the  sedi- 
mentary aspect  ol  the  granites, 
and  the  belief  that  all  crystalline 
schists  were  Archean,  gave  rise 
to  the  belief  that  they  are  of 
great  age. 

The  gneisses  of  the  Alps  occur  in  contact  with  a  great  variety  of 
rocks  belonging  to  relatively  recent  formations,  from  Carboniferous 
to  Jurassic,  as  shown  by  their  petrographic  character  or  their 
fossils.  In  some  cases  several  superposed  beds  of  gneiss  occur  in 
the  sediments,  in  others  the  latter  are  cut  by  the  gneiss.  Further, 
fragments  of  the  sediments  occur  in  the  gneiss,  and  fragments  of 
the  gneiss  in  altered  sediments.  All  this  is  shown  in  Fig.  104,  which 
represents  one  of  the  best-known  occurrences,  the  contact  of  gneiss 
and  limestone  in  the  Bernese  Oberland.  If  the  gneiss  is  Archean, 
it  is  much  older  than  the  overlying  and  interbedded  sediments, 
consequently  its  position  can  be  explained  only  by  assuming  that 
great  displacements,  faults,  and  folds  have  brought  these  rocks  to 
their  present  positions.  Believers  in  the  theory  of  dynamometa- 
morphism  found  in  these  hypothetical  movements  the  cause  for 
the  alterations  in  the  rocks,  but  it  is  much  more  probable  that 
they  are  due  exclusively  to  contact-metamorphism. 


FIG.  103. — Limestone  with  kneaded 
structure.  Lochseite  near  Schwanden, 
Switzerland. 


172         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

Even  after  petrographic  investigations  showed  that  there  could 
be  no  question  as  to  the  original  granitic  character  of  these  rocks, 
the  idea  of  great  age  still  clung  to  them,  and  was  expressed  in  the 
name  protogine.  The  contact  between  the  granite  and  the  schist 
was  supposed  to  have  been  caused  by  later  displacements.  The 
granite  was  thought  to  have  possessed  originally  a  normal,  hap- 
hazard texture,  but  under  the  stresses  of  faulting  and  folding  to 
have  developed  such  new  minerals  as  occupy  the  smallest  possible 
molecular  volumes,  for  example,  clinozoisite  and  garnet.  At 
the  same  time,  the  originally  unoriented  mica  flakes  arranged 
themselves  at  right  angles  to  the  direction  of  the  pressure. 


Limestone  Gneiss 

FIG.  104. — Contact  of  gneiss  and  limestone  in  the  Bernese  Oberland. 

Great  quantities  of  anomalous  minerals  occur  in  these  rocks  as 
irregularly  distributed  inclusions  in  the  plagioclase  crystals.  The 
latter  are  usually  absolutely  fresh,  and  if  the  mica  flakes  rotated 
within  them,  it  is  necessary  to  assume  that  the  plagioclase  was  in 
a  viscous  condition.  As  a  result,  believers  in  the  theory  of  dyna- 
mometamorphism  found  it  necessary  to  postulate  that  the  origin- 
ally haphazard-textured,  normal  granite,  which  crystallized  from 
the  melt,  was  reduced  by  later  orogenic  forces  to  a  condition  very 
similar  to  that  of  the  original  magma,  and  that  out  of  this  mass  it 
crystallized  for  the  second  time,  but  now  as  gneiss. 

According  to  this  theory  of  secondary  schistosity  of  the  granite, 
the  metamorphism  of  the  sedimentary  rocks  which  overlay  them 
proceeded  under  the  same  conditions.  Since  the  granite  was 
assumed  to  be  of  greater  geologic  age,  the  sediments  must  have 


,.        REGIONAL  METAMORPHISM  173 

been  deposited,  later  upon  the  previously  eroded  granite,  or  have 
been  folded  into  or  thrust  over  it. 

That  the  central  gneiss  has  broken  through  its  schist-shell  in  numerous  cases, 
that  magmatic  differentiation  is  shown  by  aplitic  and  lamprophyric  phases  hi  the 
border  zones,  and  that  the  aplitic  dikes  succeeding  the  granitic  intrusion  cut  not  only 
the  granite  massif  -tself  but  also  the  schistose  mantle  to  a  distance  of  many  kilometers, 
all  these  were  disregarded  by  believers  in  this  theory,  or  at  most  were  used  by  them  as 
bases  for  constructing  artificial  faults  and  folds,  which  were  then  offered  as  further 
proofs  of  dynamometamorphism. 

That  no  typical  contact-minerals,  such  as  are  always  present  elsewhere  in  contact- 
zones,  occur  in  the  mica-schists  or  the  limestones  of  the  Alpine  region  has  been  cited 
again  and  again  as  strong  evidence  against  the  theory  that  the  schistose  crystalline 
border  of  the  granite  is  due  to  contact-metamorphism.  This  objection  is  easily  met, 
and  reference  need  only  be  made  to  the  section  on  piezo-contact-metamorphism, 
where  various  views  are  brought  together. 

Facies  of  the  Crystalline  Schists. — If  all  the  rocks  which  have 
been  called  crystalline  schists  in  different  regions  are  examined 
more  closely,  two  end-members  connected  by  various  transitions 
may  be  recognized.  It  is  to  be  emphasized,  however,  that  these 
transition  rocks  do  not  unite  the  two  extremes  in  one  and  the  same 
area,  but  that  the  end-members  are  always  sharply  separated. 

In  almost  every  occurrence  it  is  possible  to  distinguish  three 
varieties  of  "  crystalline  schists." 

1.  Igneous  rocks  of  normal  composition  which  show  schlieren 
or  a  schistose  development.     These  are  called  ortho-rocks. 

2.  Metamorphosed   rocks,   best   grouped   together   as   meta- 
morphic  schists  or  para-rocks. 

3.  A  combination  of  both  rocks.     In  these  the  metamorphic 
schists  are  saturated  with  the  igneous  rock.     They  are  injection- 
schists  or  migmatites,  and  may  be  called  meta-iocks. 

With  these  subdivisions  in  mind,  an  examination  of  the  parallel- 
textured,  unaltered,  igneous  rocks  of  the  intermediate  Alps  shows 
them  to  be  chiefly  granites  whose  composition  and  structure  have 
been  somewhat  modified  by  inclusion  of  schistose  rocks.  In  the 
corresponding  rocks  of  the  central  Alps,  and  in  the  cores  of  folded 
mountain  ranges  in  general,  the  parallel  structure  appears  to  be 
the  result  of  piezocrystallization,  and  these  rocks  clearly  show  the 
action  of  pressure  in  their  structure  and  mineralogic  composition. 
In  general,  therefore,  a  distinction  can  be  made  between  a  normal 
and  an  Alpine  facies,  the  end-members  being  quite  distinct. 

Further,  it  is  significant  that  corresponding  to,  and  connected 
with,  these  contrasting  facies  of  gneiss,  there  are  also  contrasting 


174         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

facies  of  metamorphosed  schists.  The  Alpine  facies  of  the  schist 
show  piezo-contact-metamorphism,  while  that  which  occurs  with 
the  normal  gneiss  has  more  or  less  the  composition  of  a  normal  con- 
tact-rock. In  other  areas  the  gneiss  as  well  as  the  metamorphosed 
schist  is  of  intermediate  character. 

On  the  theory  of  dynamometamorphism,  these  different  facies  are  explained  as 
phenomena  of  alteration  dependent  upon  depth.  Three  zones  are  differentiated. 

1.  The  upper  zone,  under  relatively  low  temperature  and  little  hydrostatic  pressure, 
on  account  of  the  comparatively  slight  weight  of  the  overlying  strata,  but  under  great 
lateral  pressure  from  the  action  of  erogenic  forces.     In  this  zone  the  re-formation  is 
predominantly  mechanical,  and  few  new  minerals  are  formed ;  those  that  do  appear  are 
hydroxyl-rich  and  of  minimum  volume.     The  syllable  epi  (Gr.  ort,  above)  is  prefixed 
to  the  names  of  the  rocks  occurring  here,  for  example,  epi-greenstone-schists,  epi- 
quartzites,  etc.     They  correspond  to  the  rocks  occurring  in  the  outer  piezo-contact- 
metamorphic  zone. 

2.  An  intermediate  zone  under  higher  temperature.     Mechanical  textures  are 
less  common,  and  complete  recrystallization  takes  place  with  the  formation  of  heavier 
and  less  hydrous  minerals.     Rocks  supposed  to  have  originated  here  receive  the  pre- 
fix meso-  (Gr.  /xco-os,  in  the  middle),  for  example,  meso-mica-schist,  mesomarble,  etc., 
and  correspond  chiefly  to  the  rocks  of  the  inner  zone  of  piezo-contact-metamorphism. 

3.  The  deepest  zone,  in  which  high  temperature  is  of  greatest  importance.     Here 
the   prefix  kata-  (Gr.  /cdrw,  below)  is  used,  as  in  katagneiss,  kataamphibolite,  etc. 
These  rocks,  in  general,  correspond  to  normal  contact-rocks,  and  to  their  injected  and 
resorbed  varieties. 

Perhaps  there  would  be  a  certain  justification  for  this  scheme  of  classification  if  it 
were  possible  somewhere,  or  in  some  manner,  to  determine  the  depth  within  the  earth's 
crust  at  which  the  metamorphism  took  place.  As  it  is,  the  depth  is  determined  solely 
by  characteristics  which  geologists  think  the  rocks  from  such  depths  should  show. 

These  divisions  would  be  acceptable  if  the  different  zones,  or  at  least  the  last  two, 
could  somewhere  be  found  in  place,  one  above  the  other.  At  present  the  rocks  have 
never  been  found  together,  the  meso-  and  kata-rocks  being  everywhere  separated  geo- 
logically, and  usually  geographically.  Although  the  meso-rocks  definitely  occupy 
an  intermediate  position  petrographically,  this  does  not  mean  that  in  any  region 
they  actually  represent  transition  members  between  the  two  extremes.  It  is 
not  a  difference  in  depth  which  has  produced  the  difference  between  these  rocks,  and 
just  as  little  are  dynamic  processes  the  chief  cause  of  their  metamorphism.  The 
variation  is  due,  with  much  greater  probability,  to  different  physical  conditions, 
such  as  were  contrasted  above,  namely,  piezocrystallization  and  piezo-contact-meta- 
morphism of  normally  crystallized  melts  on  the  one  hand,  and  normal  contact-meta- 
morphism  on  the  other.  The  processes  of  regional  metamorphism,  which  are  appar- 
ently so  extensive,  greatly  resemble  in  their  essentials  those  of  simple  contact- 
metamorphism. 

Summary. — A  broad  view  of  all  dynamometamorphism,  at 
least  in  the  upper  and  middle  zones,  may  be  obtained  by  examples 
from  the  central  Alps.  The  central  Alpine  gneisses  (central  gneiss, 
protogine,  etc.)  are  chiefly  granitic  rocks  which  differ  from  normal 
granites  in  containing  certain  accessory  constituents,  and  by  certain 


REGIONAL  METAMORPHISM  175 

peculiarities  of  structure.  They  occur  in  contact  with  a  schistose 
mantle  of  Carboniferous  or  Jurassic  age,  as  shown  in  a  few  cases 
by  fossils.  The  border  zone  is  composed  of  rocks  which  are  very 
different  in  different  parts  of  the  Alps  but  which  possess  the  com- 
mon characteristic  that  normal  contact-minerals  are  more  or  less 
completely  wanting.  The  relationships  between  the  igneous  rock 
and  the  surrounding  schist-zone,  however,  are  such  that  they  can 
be  explained  only  on  the  basis  of  a  primary  igneous  and  not  a 
secondary  mechanical  contact.  In  other  words,  these  gneiss-like 
granites  solidified  within  the  schistose  shell  in  which  they  now 
occur.  They  are  not  Archean,  but  clearly  younger  than  the 
surrounding  sediments  into  which  they  were  forced  in  a  molten 
condition. 

The  cause  for  the  enormous  flows  which  must  have  occurred  in  the  whole  central 
chain  of  the  Alps  in  relatively  recent  geologic  periods  probably  can  be  found  in  con- 
siderable faulting  during  the  period  of  especially  great  folding.  During  such  erogenic 
movements  the  rocks  were  completely  shattered,  thereby  making  easy  paths  for  the 
melt  in  its  movement  upwards.  The  magma  finally  crystallized  under  great  stress. 
At  the  same  time,  the  mineralizers  which  emanated  from  the  magma  saturated  the 
shaken  country-rock  to  unusual  distances  and,  on  account  of  the  exceptionally  high 
pressure,  produced  molecular  re-arrangements  which  led  to  the  formation  of  rare 
mineral  combinations. 

This  view  offers  an  explanation  for  the  various  geologic  modes  of  occurrence  of  the 
central  Alpine  gneisses,  and  for  their  petrographic  characteristics;  why  gneiss-like 
granites  occur  almost  everywhere  with  the  schists,  and  why  the  crystallinity  of  the 
schists  generally  decreases  with  distance  from  the  gneiss.  It  is  especailly  noteworthy 
that  in  the  Low  Tauern  the  rocks  farthest  from  the  central  granite  are  very  much 
more  faulted  than  those  nearer  to  it,  yet  they  are  much  less  crystalline.  Further, 
some  of  the  crystalline  chloritized  schists  which  directly  overlie  the  central  granite 
contain  remarkably  well-preserved  plant  remains.  The  common  slates,  however, 
a  little  farther  away,  are  intensely  folded,  faulted,  and  in  many  cases  transversely 
schistose,  and  while  their  sedimentary  character  has  not  been  altered  by  molecular 
re-arrangement,  their  fossil  plants  appear  only  in  indistinct  traces. 

Even  according  to  Rosenbusch,  the  textures  of  sediments  altered  by  dynamometa- 
morphism  cannot  be  distinguished  from  the  textures  of  contact-rocks.  In  both, 
the  minerals  become  filled  with  inclusions  arranged  parallel  to  the  original  schistosity, 
and  the  coarser  clastic  material  and  the  sharp  boundaries  between  the  different  beds 
appear  as  clearly  in  the  one  as  in  the  other.  Furthermore,  at  the  contact  between 
the  central  granite  and  the  limestone  there  are  developed  transition  members  analo- 
gous to  those  found  along  normal  igneous  contacts  against  limestone,  for  example 
at  Monzoni,  Christiania,  etc.  Again,  tourmaline  appears  everywhere  as  a  metaso- 
matic  mineral  in  the  rocks  of  the  schist  zone  of  the  central  Alps,  and  its  long,  needle- 
like  crystals,  in  many  cases  embedded  at  right  angles  to  the  schistosity,  show  the  same 
bands  of  inclusions  as  do  the  remaining  constituents.  It  is  very  noteworthy  that 
in  many  rocks  of  the  schist-zone,  even  in  those  which  externally  appear  intensely  folded, 
no  trace  of  mechanical  deformation  appears;  the  mica  flakes  are  not  bent  nor  the  tour- 
maline cyrstals  broken.  To  explain  the  peculiar  phenomenon  that  minerals  which 
originated  by  orogenic  processes  show  no  traces  of  deformation  by  these  forces,  Rosen  - 
12 


176         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

busch  formulated  the  law  that  no  mineral  can  be  destroyed  by  the  force  which  formed 
it.  Since  it  is  inconceivable  that  a  crystal  which  originated  through  dynamometa- 
morphism  should  behave  in  a  different  manner  under  pressure  from  another  crystal 
of  the  same  substance  but  formed  under  different  conditions,  this  law  should  be  dis- 
carded as  unjustifiable. 

Little  is  to  be  gained  by  giving  special  names  to  the  textures  produced  by  dynamo- 
metamorphism.  If  the  larger  porphyritic  crystals  of  such  rocks,  usually  with  irreg- 
ular outlines  and  corresponding  to  the  "knots"  of  normal  contact-rocks,  are  called  por- 
phyroblasts  (Gr.  /SXaoros,  sprout),  it  no  more  suggests  their  special  chemico-geological 
significance  than  if  the  normal  mosaic  texture  of  contact-rocks,  which  is  also  found 
everywhere  in  the  crystalline  schists,  is  called  granoblastic.  The  appearances  are 
identical  whatever  the  names,  and  this  was  clearly  recognized  by  Rosenbusch  himself, 
the  most  prominent  among  the  founders  of  dynamometamorphism. 

Granite  and  the  Crystalline  Schists. — Taken  all  in  all,  the 
geologic  as  well  as  the  petrographic  attributes  of  the  crystalline 
schists  of  the  central  Alps  indicate  an  intimate  connection  between 
the  igneous  origin  of  the  central  granite  and  the  contact-metamor- 
phism  of  the  country-rocks.  In  consequence  of  the  general 
shattering  of  the  rocks  by  orogenic  movements,  the  metamorphic 
contact-zone  may  be  traced  to  unusually  great  distances  from  the 
central  massif,  in  many  cases  to  double  the  normal  distance.  In 
many  places  at  these  great  distances,  definite  proof  of  igneous 
activity  is  offered  by  the  occurrence  of  aplite  dikes.  It  would  have 
been  impossible  for  the  aplite  to  have  developed  a  holocrystalline 
texture  except  where  the  country-rocks  themselves  were  still  warm 
enough  to  permit  the  slow  cooling  of  the  intruded  material. 

The  action  of  great  pressure  is  very  distinctly  shown  by  the 
condition  of  the  various  crystalline  schists  of  the  Alps,  and  it  is 
primarily  in  the  relations  between  these  rocks  and  those  which  are 
undoubtedly  igneous  that  the  causes  for  their  abnormalities 
are  to  be  looked  for.  The  orogenic  processes  shattered  the 
rock  and  the  magma  of  the  deeps  was  forced  into  the  loosened 
strata,  there  to  undergo,  because  of  continuing  stresses,  the 
numerous  modifications  which  are  included  under  the  name  of 
piezocrystallization. 

The  gneiss-like  appearance  and  the  abnormal  accessory  con- 
stituents of  these  granites,  therefore,  become  primary  properties. 
Judging  from  their  similarity — chemical,  mineralogical,  and  struc- 
tural— these  rocks  appear  to  be  of  about  the  same  geologic  age 
in  the  whole  chain  of  the  Alps,  and  this,  certainly,  relatively  recent. 
As  was  shown  above,  the  contact-metamorphism  of  the  country- 
rock  did  not  begin  until  the  granite  was  in  the  last  stages  of  its 


REGIONAL  METAMORPHISM 


177 


solidification.  The  great  stresses  which  still  existed  everywhere 
within  the  disturbed  region  are  shown  in  the  anomalous  minerals 
of  the  piezo-contact-metamorphism;  that  the  usual  mechanical 
textures  do  not  everywhere  appear  is  due  to  the  fact  that  the 
movements  of  the  crust,  for  the  most  part,  came  to  a  standstill 
with  the  solidification  of  the  great  granite  mass.  Entirely 
analogous  phenomena  may  be  observed  in  all  other  greatly-folded 
regions,  and  everywhere  piezocrystallization  and  piezo-contact- 
metamorphism  go  hand  in  hand. 

The  character  of  the  crystalline 
schists  in  undisturbed  regions  is  ex- 
tremely variable,  and  only  excep- 
tionally can  piezocrystallization  be 
recognized.  Where  this  is  the  case, 
a  more  or  less  perfect  parallelism  of 
the  mica  flakes,  due  to  the  forma- 
tion of  schlieren  in  the  border  zone 
where  schistose  material  was  assimi- 
lated, can  be  observed  in  rocks 
which  in  texture  and  otherwise  are 
normal  granites.  By  no  effort  of 
the  imagination  can  these  rocks  be 
regarded  as  having  been  formed  by 
dynamometamorphism,  since  they 
show  no  trace  of  a  cataclastic  tex- 
ture. Neither  are  the  much-folded 
cordierite-gneisses  of  the  Bavarian 

Forest  dynamometamorphosed,  but  they  are  built  up  of  parallel 
bands  of  rocks  of  the  most  different  types.  Bands  of  cordierite- 
hornfels,  the  most  normal  of  contact-rocks,  alternate  with  granite- 
aplite.  The  latter  are  of  considerable  size  in  the  troughs  and  crests 
of  the  folded  schist,  and  in  many  cases  cut  it  transversely,  yet  the 
constituents  of  these  rocks  and  of  the  hornfels  show  neither  inter- 
nal bending  nor  fracturing.  As  a  matter  of  fact,  the  cordierite- 
gneisses  of  the  Bavarian  Forest  are  normal  injection-schists,  with 
no  traces  of  mechanical  action,  in  contact  with  normal  granitic 
rocks. 

The  thickness  of  the  crystalline  schists  in  different  regions  is  very  variable,  the 
maximum  estimate  being  50  to  60  km.  A  critical  examination  of  the  different  areas 
shows  that  instead  of  forming  a  geologic  unit,  the  schist  is  actually  composed  of  many 
different  members.  The  sketch  map  in  Fig.  105  shows  a  part  of  the  so-called  Her- 


>s 


Gneiss       Granite    Mica-schist       Pfal 

FIG.  105. — The  gneiss  region  of  the 
inner  Bavarian  Forest. 


178         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

cynian  gneiss  formation  of  the  Bavarian  Forest.  Taking  into  consideration  the  dip 
of  the  strata,  the  thickness  is  about  12  to  15  km.  A  cross-section  through  the  region, 
for  example  near  Zweisel,  shows  at  least  ten  large  and  innumerable  smaller  areas 
of  true  haphazard  granite,  in  all  certainly  amounting  to  more  than  5  km.  These 
5  km.  must  therefore  be  subtracted  from  the  total  thickness  of  the  complex.  Further, 
the  large  granite  masses  show  that  a  great  granite  massif  is  present  beneath  the  whole 
region,  the  insignificant  portions  laid  bare  by  erosion  being  simply  apophyses  from  it. 
The  granite  not  only  occurs  in  these  larger  masses  but  it  is  injected  into  the  schists 
everywhere  in  the  region.  This  shows  that  the  15-km.-wide  altered  zone  is  only  seem- 
ingly the  extent  of  the  metamorphism,  the  sedimentary  rocks  themselves  actually 
being  in  much  more  intimate  contact  with  the  granite  than  is  apparent  at  the  surface. 
Toward  the  north,  near  Eisenstein,  the  granite  impregnation  ends,  and  only  isolated 
patches  of  granite  are  found.  The  rock  here  is  mica-schist,  while  still  farther  distant 
it  is  phyllite.  As  the  distance  from  the  igneous  rock  increases,  the  folding  and  crum- 
pling of  the  schist  decreases,  the  so-called  phyllites  showing  only  very  fine  crinkling. 

If  the  rocks  which  have  been  altered  by  contact-metamorphism, 
and  this  includes  most  of  the  gneiss,  be  subtracted  from  the 
enormously  thick  crystalline  schist-formation,  there  will  remain 
but  a  small  portion  of  the  rocks  to  be  included  in  the  estimate  of 
thickness.  It  must  be  remembered,  also,  that  where  narrow, 
uniform  dikelets  of  igneous  rocks  intersect  a  region,  they  can 
only  be  apophyses  of  larger  masses  lying  underneath.  With  these 
facts  in  view,  the  distinction  between  contact-  and  regional-meta- 
morphism,  which  always  has  been  emphasized  in  geologic  litera- 
ture, loses  much  of  its  significance,  and  it  becomes  still  less  im- 
portant when  it  is  noted  that  the  most  characteristic  rocks  of  con- 
tact-zones, such  as  "knoten-,"  "garben-,"  and  chiastolite-schists, 
also  occur  in  the  crystalline  schist  area.  The  identical  textures 
and  mineralogic  compositions  of  innumerable  crystalline  schists 
and  contact-rocks  show  the  analogy  between  the  alteration 
processes  by  which  they  were  formed.  If  all  the  gneisses  which 
have  been  shown  by,  modern  petrographic  methods  to  be  of 
igneous  origin,  and  all  those  rocks  which  can  be  recognized  as  true 
injection  schists,  and  finally  all  rocks  which  are  demonstrably 
due  to  contact-metamorphism  were  separated  from  the  crystalline 
schist  formation,  it  would  be  much  easier  to  grasp  the  meaning  of 
the  chemico-geologic  processes  which  produced  them  than  now 
when  all  these  different  varieties  of  rocks  are  dragged  along  as 
useless  ballast. 

Viewed  from  the  standpoint  of  StiibeFs  theory  of  vulcanism,  the  following  conclu- 
sions are  reached.  The  original  crust  and  the  oldest  sediments  can  never  be  examined, 
for  they  were  repeatedly  broken  up  by  volcanic  action,  covered  by  igneous  material, 
were  resorbed,  or  lie  buried  at  such  depths  that  the  deepest  borings  or  mines  cannot 


REGIONAL  METAMORPHISM  179 

reach  them.  An  examination  of  the  sediments  and  the  fossils  from  the  Cambrian 
to  the  present  shows  that  the  crystalline  schists  are  not  especially  old.  Variations 
in  climate  had  already  been  established  in  the  oldest  geologic  periods,  and  the  interior 
heat  had,  at  most,  only  a  very  slight  effect  upon  conditions  on  the  surface;  that  is, 
the  crust  at  that  time  had  already  attained  a  considerable  thickness.  Long  periods 
with  conditions  favorable  for  the  existence  of  organisms  preceded  the  Cambrian, 
at  which  time  the  climatic  conditions  upon  the  earth  were  nearly  the  same  as  those 
which  exist  at  the  present  time. 

The  struggle  between  igneous  and  sedimentary  formations  had  been  going  on 
for  many  long  geologic  periods  before  the  deposition  of  the  oldest  sedimentaries  now 
known.  The  deep  deposits  which  had  been  laid  down  upon  the  first  solid  crust  of 
the  earth  had  been  completely  broken  up,  and  impregnated  and  altered  by  the  igneous 
rocks,  and  from  their  fragments  new  beds  were  formed.  The  enormous  igneous 
intrusions  of  the  past  probably  altered  the  sediments  very  much  more  extensively, 
but  in  a  manner  analogous  to,  that  in  which  the  later  rocks  were  altered.  There  are 
thus  found,  in  the  oldest  clastic  deposits,  fragments  of  much  older  formations  which 
have  the  characteristics  of  the  crystalline  schists. 

No  certain  proof  exists  that  the  crystalline  schist  boulders  which  occur  in  sedimen- 
tary rocks  were  derived  from  directly  underlying  crystalline  schists,  nor  that  they 
were  already  crystalline  at  the  time  of  the  formation  of  the  sediments.  For  example, 
from  boulders  of  gneiss  in  certain  altered  sediments  of  the  central  Alps,  the  conclusion 
has  been  drawn,  without  any  attempt  to  establish  the  identity  of  the  two  kinds  of 
gneiss  petrographically,  that  the  sediments  were  deposited  upon  the  gneiss-like  granite 
after  its  intrusion.  The  sediments  were  undoubtedly  deposited  upon  some  kind  of 
a  foundation,  and  it  is  probable  that  this  foundation  was  composed,  in  part  at  least, 
of  gneiss-like  rocks.  It  cannot  be  questioned,  however,  that  the  gneiss-like  granite 
is  younger  than  the  schist  which  it  has  penetrated  and  lifted.  While  rocks  having  the 
chemical  compositions  of  argillites  and  sandstones  indicate  the  previous  weathering 
and  denudation  of  a  region  of  granite  or  gneiss  just  as  clearly  as  do  included  boulders, 
yet  one  is  not  justified  in  concluding  from  the  occurrence  of  sandstone  or  arkose  in 
the  beds  overlying  a  granite  that  they  were  formed  from  the  weathering  of  the  latter. 

In  conclusion,  the  results  of  these  observations  may  be  sum- 
marized as  follows: 

1.  The  crystalline  schist  formation  is  not  a  formation  in  a 
geologic  sense,  for  certain  more  recent  rocks  have  all  the  petro- 
graphic  characters  of  the  crystalline  schists.     Their  petrographic 
similarity,  which  is  the  only  reason  for  placing  them  all  in  the 
so-called  Archean,  can  not  be  used,  therefore,  to  determine  their 
geologic  age.     The  rocks  which  at  the  present  time  are  classed  as 
older  or  true  crystalline  schists,  and  are  placed  in  the  Archean 
formation,  may  be  of  any  geologic  age;  the  only  property  common 
to  all  being  the  fact  that,  as  yet,  no  fossils  have  been  found  in 
them,  the  younger  crystalline  schists  differing  from  them  only 
in  this  respect. 

2.  The  crystalline  schist  formation  is  not  a  formation  in  a 
petrographic  sense,  its  rocks  having  different  modes  of  origin. 


180         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

They  are  in  part   sediments,   in  part  igneous  rocks;  some^  are 
primary,  some  are  metamorphosed. 

3.  The  so-called  crystalline  schists  are  not  necessarily  meta- 
morphic  rocks,  for  some  of  them  have  not  been  metamorphosed. 

4.  Many  extensive  areas  of  crystalline  schists  are  made  up  of 
igneous    and    contact-metamorphosed    rocks    of    very    different 
geologic  ages.     Instead  of  calling  the  different  members  mica- 
schist,  etc.,  names  which  give  false  impressions  as  to  their  ages, 
they  should  be  designated  by  proper  descriptive  names.     Only 
after  being  thus  separated  will  it  be  possible  to  determine  the 
genetic  relationships  of  true  crystalline  schists. 

5.  Dynamic  disturbances  primarily  produce  brecciation  of  the 
constituents  of  a  rock.     Whether  they  may  give  rise  also  to  mo- 
lecular re-arrangements  cannot  be  determined  definitely.     Most  of 
our  knowledge  at  the  present  time  seems  to  indicate  the  contrary. 

6.  The  theory  of  dynamometamorphism  can  no  more  withstand 
a  critical  examination  than  can  the  theories  of  the  formation  of 
primary   crystalline   schists,    diagenesis,    or   plutonic    or  hydro- 
chemical  regional  metamorphism. 

7.  The  oldest  known  fossiliferous  sediments  were  formed  under 
conditions  which  differed  at  most  but  very  slightly  from  those 
existing  at  the  present  time.     From  the  first  formation  of  the 
earth  to  the  deposition  of  the  Cambrian,  therefore,  much  more 
time  must  have  elapsed  than  is  generally  allowed  by  geologists. 
The  Cambrian  must  have  been  preceded  by  long  periods  of  sedi- 
mentation, but  no  more  traces  of  these  deposits  have  been  found 
than  of  the  still  older  chemical  precipitates  from  the  hot,  primordial 
universal  sea. 


X.    JOINTING  AND  TEXTURES 

LITERATURE 

F.  BERWERTH:  " Mikroskopische  Strukturbilder  der  Massengesteine."    Wien,  1895- 

1900. 
CROSS,  IDDINGS,  PIRSSON,  AND  WASHINGTON:  " The  Texture  of  Igneous  Rocks."    Jour. 

Geol.,  XIV  (1906),  692. 
J.  P.  IDDINGS:  "The  Crystallization  of  Igneous  Rocks."     Bull.  Phil.  Soc.  Washington, 

XI  (1889),  65. 

A.  MICHEL-LEVY:  "Structure  et  classification  des  roches  eruptives."  Paris,  1889. 
H.  ROSENBUSCH:  "tlber  das  Wesen  der  kornigen  und  porphyrischen  Struktur  bei 

Massengesteinen."     Neues  Jahrb.,  1882,  II,  11. 
IDEM:  "tjber  Struktur  und   Klassifikation  bei   Eruptivgesteinen."     Tscherm.  min. 

petr.  Mitt.,  XII  (1891),  351. 
W.   SALOMON:  "tJber    Gestemskliiftung    und    Kliiftbarkeit."     Der   Steinbruch,  VI 

(1911),  227. 
A.   SAUER:  "Mikroskopische   Strukturbilder  wichtiger  Gesteinstypen."     Stuttgart, 

1906. 
H.  C.  SORBY:  "On  the  Microscopical  Structure  of  Crystals  Indicating  the  Origin  of 

Minerals,  and  Rocks."  ^Quart+J'our.  Geol.  Soc.,  XIVv(1858),  433. 
F.  ZIRKEL:  "Mikrpskopische  Struktur  der  Gesteine."     Pogg.  Ann.,  CXIX  (1863), 

288. 

+ 

Appearance   of   Surface   Exposures   of  Various  Rocks. — The 

general  character  of  the'"  surf  ace  exposures  of  any  type  of  rock 
depends  primarily  upon  its  jointing  and  its  texture.  A  rock  mass 
is  usually  divided  into  separate  parts  by  joint-planes  which  may 
be  open  or,  in  fresh  rocks,  hardly  visible.  Entering  the  rock  along 
these  joints,  the  agents  of  weathering  proceed  to  destroy  it,  the 
amount  and  rapidity  of  the  destruction  depending  primarily  upon 
the  texture  of  the  rock  and  the  number  of  joints. 

The  relief  of  the  surface  is  not  produced  exclusively  by  these 
factors,  however,  for  the  various  climatic  zones  of  weathering  are 
of  great  importance,  and  one  and  the  same  rock  may  assume  quite 
a  different  appearance  in  a  different  zone.  Granular  igneous  rocks, 
which  undergo  chemical  weathering  relatively  easily  and  are  con- 
verted into  grush  by  partial  solution,  show  soft,  rounded  outlines 
with  gentle  slopes  in  regions  in  which  chemical  weathering  pre- 
dominates. This  is  the  normal  mode  of  occurrence  of  granite 
in  moist  temperate  or  warm  climates  (Fig.  106).  But  the  same 
rock  takes  on  an  entirely  different  form  in  arid  regions.  Here  in 

181 


182         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

many  cases  it  is  exposed  in  vertical  walls  (Fig.  107),  and  presents 
a    very    impressive    appearance.     There    is    a  similar  difference 


FIG.  106. — Rounded  exposures  of  granite  with  a  rock-sea  in  the  foreground.     Grosse 
Schneegrube,  Riesengebirge-.     (H.  Eckert,  Prag,  Photo.) 

between  argillites,   sandstones,  etc.  under  these  conditions,  for 
while  they  disintegrate  readily  in  humid  regions  and  rarely  out- 


FIG.  107. — Granite  walls,  Mount  Sinai.     (After  v.  Lendenfeld.) 

crop,  in  arid  regions  they  stand  out  in  most  grotesque  shapes  as 
hoodoos,  buttes  (Fig.  51),  and  other  bad-land  forms. 


JOINTING  AND  TEXTURES 


183 


Rocks  which  have  no  regular  joint-planes  but  are  quite  compact, 
especially  serpentine  (Fig.  108),  reef-limestones  and  dolomites 
(Fig.  55),  quartz-dikes  (Fig.  56),  etc.  tend  under  all  conditions  to 
stand  out  in  distinct  relief  from  their  surroundings. 


FIG.  108. — Serpentine  stock.     Goslerwand,  Gross  Venedig  province. 

But  it  is  not  alone  the  firmly-cemented  and  jointless  rocks  which 
appear  in  steep  cliffs;  these  forms  are  just  as  characteristic  in  mas- 
sive deposits  of  loose,  fine-grained  materials  which  weather  readily. 


FIG.  109. — Trass  deposits  in  the  Brohltal  near  Andernach  on  the  Rhine. 

Examples  may  be  found  in  the  cliffs  of  the  Appenines,  along  the 
chalk  shores  of  the  Baltic  and  North  Seas,  in  the  trass  deposits 
in  the  Brohl  Valley  (Fig.  109),  and  in  the  loess  deposits  of  central 


184 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


Asia.  Just  as  characteristic  are  the  so-called  earth  pillars  or 
hoodoos  (Fig.  63)  which  are  found  in  many  porous  glacial  deposits, 
talus  slopes,  and  volcanic  tuffs. 

If  the  characteristic  forms  of  rocks  which  possess  distinct  joint- 
ing are  examined,  it  will  be  seen  that  there  is  a  close  relationship 
between  these  forms  and  the  forms  of  the  mountains  themselves. 
Where  normal  granites  have  approximately  horizontal  joint- 
planes,  they  appear  in  the  usual  rounded  exposures,  but  where  the 
schistosity  is  good  and  is  at  a  high  angle,  the  rock  appears  in  sharp 
ridges  and  ragged  peaks  (Fig.  110).  ,Various  sedimentary  rocks, 


FIG.  110. — Jagged  peaks  in  schistose  granite  (protogine).     Aiguilles  des  Charmoz 
et  de  Trelaporte.     (Wehrli,  Zurich,  Photo.) 

here  and  there,  show  similar  ragged  features  when  their  bedding- 
planes  are  steeply  inclined.  Where  the  strata  are  more  nearly 
horizontal  the  exposures  are  flat,  and  the  planes  may  be  deeply 
dissected  by  canyons  (Fig.  53). 

Jointing  and  Parting  in  Rocks. — The  terms  jointing  and  parting 
are  applied  to  certain  directions  in  rocks  in  which  they  fracture 
more  easily  than  in  others.  In  this  sense  joint-planes  include  the 
bedding-planes  of  sedimentary  rocks  as  well  as  certain  fracture- 
planes,  called  rift  and  grain  by  quarrymen,  of  many  igneous  rocks. 
The  latter  lines  of  weakness,  which  may  be  imperceptible  to  the 
inexperienced  in  rocks  which  are  fresh  but  distinctly  visible  when 


JOINTING  AND  TEXTURES 


185 


they  are  weathered  (Figs.  6  and  47),  permit  the  removal  of  rec- 
tangular blocks  with  relatively  plane  faces  from  quarries  in  which 
the  rocks  appear  to  be  perfectly  massive.  The  stresses  producing 
joints  of  this  kind  were  developed  by  the  contraction  of  the  cooling 
igneous  rock.  They  must  have  been  of  extraordinary  force  at 
times,  for  the  hardest  and  least  cleavable  minerals,  such  as  quartz 
and  olivine,  have  in  some  cases  been  so  perfectly  and  smoothly 
sheared  that  the  two  parts  of  a  crystal  may  be  found  in  opposite 
sides  of  a  fissure. 

The  most  common  method  of  separation  in  igneous  rocks  is  the 
so-called  platy-parting,  which  predominates  in  persilicic  and 
mediosilicic  rocks  of  the  granite,  quartz-porphyry,  phonolite,  and 


FIG.  111.— Platy-parting 


laccolith.     Flossenbiirg,  near  Weiden,  Oberpfalz. 


andesite  groups.  This  parting  is  generally  parallel  to  the  cooling 
surface;  in  granite  (Fig.  Ill),  for  example,  it  is  parallel  to  the 
surface  of  the  laccolith,  while  in  extrusive  rocks,  such  as  phonolite, 
it  is  parallel  to  the  surface  of  the  lava-flow.  In  vertical  dikes, 
the  parting  is  vertical,  and  the  joints,  parallel  to  the  cooling  sur- 
face, may  give  rise  to  gorges  with  vertical  walls,  as  in  the  Eggental 
(Fig.  112). 

The  behavior  of  different  rocks  with  platy-parting,  when  sub- 
jected to  weathering,  is  quite  different.  The  plates  of  fissured 
granites  are  only  slightly  thinner  near  the  surface  than  farther 
down,  while  phonolite,  under  the  £ame  conditions,  splits  into 
plates  no  more  than  a  millimeter  in  thickness,  the  so-called  paper- 


186 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


porphyry  (Fig.  49).     Plates  of  quartz-porphyry  generally  differ 
but  little,  whether  fresh  or  weathered. 


FIG.  112. — Platy -parting  in  quartz-porphyry.     Eggental,  Bozen. 

Second    in    importance    is    columnar   jointing.     This    occurs 
primarily  in  subsilicic  extrusive  rocks  such  as  trap   (Fig.   113), 


FIG.  113. — Prismatic  parting  in  trap.     Giant's  Causeway,  Ireland.     (After  F.  Toula.) 

melaphyre,  and  basalt,  but  it  is  also  found  in  silicic  rocks,  for 
example  in  the  quartz-porphyry  from  Sigmundskron,  near  Bozen. 


JOINTING  AND  TEXTURES 


187 


Upon  weathering,  such  columns  develop  cross-joints,  and  gradually 
exfoliate  into  onion-like  spheres  (Fig.  48).     In  other  cases  oblique 


FIG.  114. — Oblique  parting  in  quartz-porphyry.     Crest  of  the  Wolf  stein,  Kosten, 
Bohemia.     (Eckert,  Frag,  Photo.) 

parting-planes  cause  the  rock  to  break  into  acute-angled  blocks 
(Fig.  114). 


t^^-^T^*^ 


FIG.  115. — Perlitic  parting.     Perlite.     Glashuttental,  Schemnitz,  Hungary. 

Glassy  igneous  rocks,  in  many  cases,  show  spheroidal  partings,  varying  from  the  mi- 
croscopic pearl-like  shells  in  perlite  (Fig.  115)  to  spheres  as  large  as  one's  fist  in  kugel- 
porphyry.  These  cracks  are  recognizable  even  in  completely  weathered  material, 
while  in  fresh  rocks  the  parting  may  be  so  complete  that  the  rock  falls  into  a  grush 
called  marecanite.  Here  and  there  distinctly  crystalline  igneous  rocks,  such  as  mela- 
phyres,  show  a  coarse  spheroidal  parting. 


188 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


Sedimentary  rocks  may  also  show  a  parting  independent  of  the  bedding.  Thus 
the  parallelopipedal  parting  of  sandstone  is  caused  by  two  very  perfect  cleavages 
at  right  angles  to  the  bedding.  Rocks  which  are  so  joined  break  up,  under  the  influ- 
ence of  weathering,  into  grotesque  columns,  such  as  those  which  make  the  scenery  of 
Saxon  Switzerland  so  attractive  (Fig.  116). 


FIG.  116. — Parallelopipedal     parting     in     sandstone.     Adersbach,     Bohemia.     (H. 

Eckert,  Prag,  Photo.) 

A  great  deal  of  parting  is  due  to  secondary  orogenic  forces.  Thus  most  of  the  gneiss 
of  the  Bavarian  Forest  is  fractured  across  the  schistosity,  while  certain  indistinctly 
schistose  granites  of  the  Oberfalz  are  so  regularly  fissured  and  recemented  by  secon- 
dary biotite,that  the  rock  appears  to  be  schistose  and  banded  in  two  directions  (Fig. 


FIG.  117. — Granite.     Wondreb,  Oberpfalz. 

117.  In  the  figure  the  secondary  cleavage,  with  its  biotite-filling,  is  horizontal). 
Similar,  but  much  more  uniform,  is  transverse  schistosity  or  rock  cleavage.  This  is 
developed  at  right  angles  to  the  pressure,  and  cuts  the  bedding  of  sedimentary  rocks 
at  an  angle.  It  is  generally  much  smoother  and  more  complete  than  the  bedding- 
plane  itself.  Such  cleavage  is  found  in  many  sandstones,  but  it  is  especially  well 


JOINTING  AND  TEXTURES 


189 


developed  in  certain  argillites  (Fig.  118),  which  become  very  compact  and  are  then  of 
commercial  importance  as  roofing-slate. 

Under  the  same  forces,  brittle  rocks  of  uniform  texture,  such  as  quartzites  or 


FIG.  118. — Transverse  schistosity  cutting  across  somewhat  bent  beds  of  slate.     Goslar. 

(Dr.  Baumgartel,  photo.) 

dolomites,  are  crushed  to  small  angular  fragments,  in  many  cases  quite  uniform  in 
size.  These  are  later  recemented  and  form  the  so-called  endogenic  (Gr.  tvSov,  within) 
breccias  (Ital.  breccia,  break)  (Fig.  119).  If  the  cement  weathers  easily,  these  rocks 


FIG.  119. — Dolomite  with  parallel  systems  of  veins.     Saalburg. 


break  down  into  an  angular  grush,  such  as  is  characteristic  of  numerous  Alpine  dolo- 
mites. 

Parting  may  also  be  brought  about  by  the  heat  of  contact-metamorphism.  This 
is  shown  in  the  columnar  parting  of  fritted  sandstones,  granites,  burnt  clays,  and  coked 
coals. 


190         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

Megascopic  Characters  of  Rocks. — The  appearance  of  a  rock, 
that  is  its  habit,  depends  upon  the  development  and  arrangement 
of  its  constituents  so  far  as  these  can  be  seen  megascopically.  In 
certain  rocks,  called  phanerites  (Gr.  Qaveds,  distinct),  the  chief 
constituents,  and  usually  their  arrangement  and  development, 
can  be  distinctly  recognized  with  the  unaided  eye  or  by  the  aid  of 
a  low-power  hand-lens.  In  other  rocks  the  greater  part  of  the  con- 
stituents cannot  be  seen  without  the  microscope.  Such  rocks  are 
megascopically  cryptomerous  (Gr.  KPUTTTOS,  hidden)  or  aphanitic 
(Gr.  d<£cm£o;ucu,  to  become  invisible). 

The  constituents  of  almost  all  rocks  may  be  recognized  by  the 
aid  of  the  microscope,  and  consequently  are  microscopic  phanerites. 
In  a  few  cases  microscopic  examination  is  of  no  assistance,  but 
with  the  development  of  microscopic  methods  the  number  of  such 
rocks  is  ever  decreasing.  Such  rocks  are  called  adiagnostic  (Gr.  «, 
not,  diayvaxris,  distinguishing). 

The  degree  of  cohesion  between  the  individual  constituents  of  a 
rock  is  very  important,  for  upon  this  depends  the  resistance  of  the 
rock  to  pressure  or  tension.  In  crystalline  rocks  the  cohesion 
depends  upon  the  more  or  less  intimate  manner  in  which  the 
individual  minerals  are  intergrown,  in  many  clastic  rocks  it  is 
doubtless  purely  mechanical,  the  overlying  pressure  having  forced 
the  individual  particles  so  closely  together  that  considerable  force 
is  necessary  to  separate  them.  The  sediments  may  be  divided  into 
three  classes,  namely  unconsolidated,  porous,  and  compact;  for 
example  sand,  clay,  and  limestone. 

As  mentioned  above  (page  6),  rocks  may  be  crystalline  or  clastic. 
The  former  may  be  subdivided  according  to  the  size  of  the  com- 
ponents into  giant-grimed,  with  constituents  ranging  in  size  from 
a  cubic  decimeter  to  several  cubic  meters,  as  in  the  pegmatites, 
Zan/e-grained  with  constituents  of  the  size  of  a  fist,  coarse-grained 
with  components  more  than  a  cubic  centimeter  in  volume,  medium- 
grained  with  individuals  of  several  cubic  millimeters,  and  fine- 
grained  with  all  components  less  than  one  cubic  millimeter  in  size. 
Dense  rocks,  finally,  are  those  which  are  megascopically  cryp- 
tomerous. By  far  the  greatest  number  of  crystalline  rocks  are 
between  medium-grained  and  dense. 

The  individual  grains  of  a  rock,  in  most  cases,  are  in  very  close  contact,  and  the 
rock  appears  compact;  but  rocks  without  minute  cavities  are  very  rare.  Fresh 
granite  and  basalt,  for  example,  are  very  compact,  yet  drops  of  moisture  may  appear 


JOINTING  AND  TEXTURES  191 

upon  freshly  fractured  faces  of  the  most  massive  basalts,  indicating  that  water  had 
access  to  the  interior.  In  some  cases  the  individual  grains  of  a  rock  are  less  intimately 
intergrown  than  in  others.  An  especially  good  example  of  this  is  found  in  saccharoidal 
dolomite,  whose  great  porosity  is  shown  by  the  rapidity  with  which  it  will  absorb 
colored  solutions.  On  account  of  its  loose  texture  it  readily  disintegrates  by  atmos- 
pheric agents  and  falls  into  the  so-called  dolomite-ash.  Granites  which  are  full  of 
little  drusy  cavities  are  porous  and  weather  very  rapidly.  They  are  called  miarolo 
by  the  Italians. 

Rocks  containing  large  angular  cavities  due  to  leaching  are  called  cellular  or  cavern- 
ous. Scoriaceous  or  vesicular  rocks  contain  round  or  elongated  cavities  produced 
by  the  escape  of  gas  from  the  melt  during  solidification.  Porous  rocks  are  those  whose 
cavities  are  chiefly  of  microscopic  dimensions.  Frothy  rocks,  finally,  are  those  made 
up  of  a  finely  divided  network  of  glassy  material,  for  example  pumice.  When  the  cavi- 
ties of  any  of  these  rocks  are  filled  with  secondary  minerals  the  rocks  are  called  amyg- 
daloids  (Gr.  anvjda\ov,  almond). 

The  constituents  of  certain  rocks  have  no  definite  orientation, 
and  the  texture  is  haphazard  or  random  (Ger.  richtungslos).     In 


FIG.  120. — Fluidal  textured  quartz-porphyry.     Gross-Umstadt,  Odenwald. 

other  rocks  the  constituents  have  a  parallel  arrangement.  Such 
rocks  are  banded  or  laminatedj  or,  when  there  is  a  distinct  parting, 
schistose. 

The  normal  texture  of  igneous  rocks  is  haphazard,  but  all 
varieties  of  parallel  textures  are  to  be  found  as  well.  Thus  in 
certain  gabbros  (Fig.  18)  a  banding  which  is  due  to  magmatic  dif- 
ferentiation shows  all  the  characteristics  of  normal  bedding.  '  A 
similar  appearance  is  found  in  many  granites  with  schlieren  (for 
example,  Fig.  98),  and  in  consequence  they  have  of  ten  been  called 

13 


192 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


gneisses.  The  latter  form  of  banding  is  especially  well  developed 
in  extrusive  rocks  which  are  predominantly  glassy,  and  since  this 
texture  originated  in  the  flowing  movement  of  the  lava  it  is  called 
fluidal  (Lat.  fluidus,  fluere,  to  flow)  or  fluctuation  (Lat.  fluctuatio, 
fluctus,  wave).  It  is  especially  distinct  in  altered  rocks,  the  parallel 


FIG.  121. — Schistose  Central  granite.     Val  Antrona,  Monte  Rosa. 

arrangement  being  brought  out  by  bands  of  different  colors  (Fig. 
120).  Elsewhere  layers  of  bubbles,  pores,  or  devitrification 
products  produce  an  excellent  banding,  or  it  may  be  caused  by 
parallel  tabular  crystals.  In  the  latter  case  a  schist-like  cleav- 
ability  is  also  developed  in  the  rocks. 


FIG.  122. — Labradorite   porphyrite   with   tabular   plagioclase.     Elbingerode,    Harz. 

As  already  mentioned,  igneous  magmas  may  take  on  a  parallel 
texture  under  the  action  of  piezocrystallization.  This  is  the  cause 
for  the  gneiss-like  character  of  the  central  Alpine  granite  (Fig.  121). 
The  flaser-  and  augen-textures  associated  with  piezocrystallization 
were  described  in  detail  on  page  56,  and  it  was  also  mentioned  that 
schistosity  may  be  produced  in  originally  haphazard  textured 


JOINTING  AND  TEXTURES 


193 


igneous  rocks    by    contact-metamorphism    or   by   post-volcanic 
processes  during  orogenic  movements. 

In  contrast  to  equigranular  rocks  are  the  porphyritic,  in  which 


FIG.  123. — Orbicular  diorite.     Santa  Lucia  di  Tallano,  Corsica.     (Prof.  Dr.  Obbeke, 

photo.) 

larger  phenocrysts  stand  out  from  a  denser  groundmass  (Fig.  122). 
Further,  there  are  spheroidal  (centric)  textures  in  both  granular 
and  glassy  rocks.  In  the  former  the  spherules  usually  consist  of 


FIG.  124. — Quartz-porphyry  with  spherulites.     Curzo,  Corsica.     (Prof.  Dr.  Obbeke, 

photo.) 

concentric  light  and  dark  shells  (Fig.  123),  in  the  latter  of  radial 
aggregates,  as  in  spherulites  (Fig.  124)  and  lithophysae.  A 
regular  or  pegmatitic  intergrowth  of  quartz  and  feldspar,  called 


194         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

graphic   granite,   may  also  be   distinctly  visible  n#  ^ascopically 
(Fig.  125). 

Sedimentary  rocks  are  typically  bedded  rocks.     They  may  be 


*  A 


FIG.  125. — Graphic  granite.     Jekaterinburg,  Urals. 

perfectly  or  imperfectly  schistose,  thin-  or  thick-bedded,  or  platy, 
and  in  some  cases,  for  example  in  limestones,  they  may  even  have 
as  haphazard  and  massive  a  texture  as  normal  igneous  rocks. 
Schistosity  parallel  to  the  bedding  is  usually  wavy  rather  than 


FIG.  126. — Ripple-marked  sandstone.     Unterschonmattenwaag,   Odenwald. 

Dr.  Klemm,  photo.) 


(Prof. 


plane,  but  these  irregularities  are  not  always  secondary.  In 
many  cases  they  are  primary,  as  in  ripple-marked  sandstone  (Fig. 
126).  In  other  places  the  original  bedding-planes  have  been 
greatly  altered  by  later  displacements,  and  folding  so  intense  that 


JOINTING  AND  TEXTURES 


195 


the  crests  aa£  troughs  are  now  very  narrow  (Fig.  127)  is  common. 
Where  alumina-poor  rocks  have  been  strongly  folded,  cross-frac- 
tures ofjnariy  kinds  are  developed,  and  these  are  filled  in  part  by 
plastic  rock-material,  in  part  by  new  deposits  of  quartz  or  feldspar. 
Furthermore  a  fine  crinkling  at  right  angles  to  the  coarser  folds  is 
developed,*  et  j/fecially  in  the  outer  contact-zones  of  plutonic  rocks. 


FIG.  127. — Folded  schist.     Alpe  Puntaiglas,  Switzerland. 

Sedimentary  rocks  may  consist  chiefly  of  coarse  quartz,  and  have 
a  coarse  clastic  texture,  a  medium  grain  as  in  sandstones,  or  a 
fine  grain  as  in  argilKtes.  Most  normal  limestones  are  dense, 
though  some  are  cellular,  and  a  cavernous  development  is  found 
in  dolomite.  Amygdaloidal  or  kidney  limestone  is  one  in  which  the 


FIG.  128. — "Kidney"  marble.     Campane  Valley,  Pyrenees'. 

calcite  occurs  in  small,  almond-shaped  grains  surrounded  by  a 
network  of  a  clay-like  substance  (Fig.  128);  where  the  calcite  is 
dissolved  out,  these  rocks  have  a  porous,  sponge-like  surface. 

Internal  Textures. — That  physico-chemical  processes  were  active 
in  the  formation  of  a  rock  is  shown  by  the  mutual  relationships 


196         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

between  the  individual  constituents  as  they  appear  in  the  textures. 
The  broad  relationships  may  be  recognized  megascopically  in 
many  cases,  but  a  detailed  study  is  only  possible  by  the  use  of  a 
microscope.  The  determination  of  the  internal  texture  is  only  a 
little  less  important  than  the  determination  of  the  rock-forming 
minerals  themselves. 

If  the  relations  between  the  textures  of  rocks  and  the  processes 
of  their  formation  can  be  determined  in  every  detail,  a  most 
important  step  forward  will  have  been  made  in  the  knowledge  of 
geochemical  processes  in  general.  At  the  present  time  the  follow- 
ing facts,  based  upon  observations,  may  be  stated: 

When  typically  developed,  three,  sharply-contrasting  types  of 
textures  may  be  distinguished.  Two  of  these  types  belong  to  rocks 
in  which  the  authigenic  constituents  greatly  predominate,  namely 
the  crystalline  rocks.  In  the  third  type,  embracing  the  clastic 
or  sedimentary  rocks,  the  constituents  are  chiefly  allothigenic. 
The  two  groups  of  crystalline  rocks  are  (1)  igneous,  and  (2)  con- 
tact-rocks and  the  crystalline  schists.  The  former  are  differen- 
tiated by  the  fact  that  the  sequence  of  crystallization  of  the  dif- 
ferent minerals  usually  can  be  easily  recognized,  by  a  characteristic 
interlocking  of  the  constituents,  by  the  relative  scarcity  of 
inclusions  in  the  minerals,  and  by  a  decided  uniformity  of  all 
their  characteristics.  The  second  group  of  crystalline  rocks,  in- 
cluding contact-rocks  and  the  so-called  crystalline  schists  which 
are  identical  with  them,  are  not  uniform  in  appearance.  The 
minerals  are  irregular  in  distribution  and  development,  they  are 
rich  in  inclusions,  and  there  is  no  determinable  sequence  of  crys- 
tallization. Finally,  some  contact-rocks  clearly  show  remnants 
of  a  different,  primary  texture,  partially  obliterated  by  the  recrys- 
tallization.  This  is  the  palimpsest  texture. 

Internal  Textures  of  Igneous  Rocks. — The  textures  of  granular 
igneous  rocks  (PL  II)  clearly  show  that  the  mobility  of  the  melts 
during  crystallization  must  have  been  considerable,  although  it 
varied  greatly  in  degree  in  different  rocks.  Igneous  rocks,  in  the 
main,  are  homogeneous  and  uniform,  and  inhomogeneities  such  as 
schlieren,  are  generally  the  results  of  differentiation  in  the  magma. 

On  account  of  the  mobility  of  the  molecules  during  crystalliza- 
tion, the  constituents,  in  many  cases,  were  able  to  unite  to  form 
large,  homogeneous  individuals.  The  boundaries  of  these  crystals, 


JOINTING  AND  TEXTURES  197 

and  various  other  features,  make  it  possible  to  read  the  history  of 
the  solidification  of  the  magma. 

The  constituents  of  igneous  magmas  crystallize  according  to 
the  laws  of  separation  from  mixed  solutions.  Since  these  laws 
depend  upon  the  external  physical  relationships  on  the  one  hand, 
and  upon  the  mutual  proportions  of  the  dissolved  substances  on 
the  other,  igneous  rocks  necessarily  show  a  variety  of  textures,  all 
characteristic,  and,  where  typically  developed,  any  one  of  them  a 
proof  of  the  igneous  origin  of  the  rock. 

Where  the  crystallization  of  a  rock  proceeded  to  its  conclusion 
uniformly  and  without  change  of  physical  conditions,  the  sequence 
of  crystallization  also  was  entirely  uniform  ;  and  the  texture  clearly 
shows  how  the  melt,  in  the  course  of  the  separation,  gradually 
became  less  complex.  Finally,  in  the  last  stage  of  the  solidifica- 
tion, the  melt  consisted  of  but  a  single  mineral  or  of  an  eutectic 
mixture  of  two  minerals,  and  this  solidified  as  a  mesostasis 
(Gr.  news,  between,  orao-is,  position)  between  the  minerals  previ- 
ously crystallized.  These  relationships  may  be  seen  distinctly  in 
Figs.  1,  2,  and  6,  PL  II.  The  mesostasis  in  Fig.  1  is  quartz,  in  Fig.  6 
augite,  and  in  Fig.  3  a  eutectic  mixture  of  quartz  and  feldspar 
developed  as  micropegmatite.  Rosenbusch1  calls  such  textures 
hypidiomorphic-granular  (Gr.  w,  nearly,  iSios,  own,  ^op^-n,  form). 


The  principal  textural  varieties  of  these  uniformly  developed  granular  and  holo- 
crystalline  (Gr.  5Xos,  entirely)  rocks  are  as  follows.  The  granitic  texture  (PI.  II,  Fig.l) 
is  one  in  which  the  dark  constituents  show  more  or  less  distinct  crystal  boundaries 
against  the  feldspars,  and  these,  in  turn,  show  crystal  boundaries  against  the  quartz. 
Further,  it  may  be  seen  that  the  less  complex  orthoclase  separated  later  than  the  more 
complex  plagioclase.  The  periods  of  separation  of  the  individual  constituents,  how- 
ever, do  not  always  follow  one  after  the  other,  but  usually  overlap.  The  crystallo- 
graphic  development  of  the  individual  constituents,  therefore,  is  rarely  so  complete 
as  that  which  is  characteristic  for  the  plagioclase  and  orthoclase  in  the  monzonitic 
texture  (PL  II,  Fig.  4). 

The  normal  sequence  of  crystallization  in  a  granite  is  shown  graphically  in  Fig.  129, 
in  which  the  abscissae  represent  the  duration  of  growth  of  the  individual  constituents. 
The  minor  accessories,  such  as  apatite,  zircon,  etc.,  which  occur  in  almost  all  rocks  in 
very  small  amounts,  are  always  the  first  products  of  separation  (I),  and  their  period 
of  crystallization  is  usually  completed  when  that  of  the  chief  constituents  of  the  rock 
begins.  Then  follows  biotite  (II),  whose  period  is  not  yet  completed  when  the  crys- 
tallization of  the  plagioclase  (III)  begins.  The  latter  overlaps  the  development 
period  of  orthoclase  (IV)  and  quartz  (V),  and  the  orthoclase  overlaps  the  quartz. 
Such  crystallization,  in  its  main  features,  is  characteristic  for  most  granites,  syenites, 
monzonites,  quartz-diorites,  and  diorites. 

1  ROHRBACH'S  term  automorphic  (1886)  has  priority  over  ROSEXBUSCH'S  idiomorphic 
(1887).  J. 


198 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


The  appearance  of  aplites  and  of  most  two-mica-granites  is  somewhat  abnormal t 
for  the  crystallization  begins  with  the  separation  of  quartz  and  usually  ends  with  the 
production  of  micropegmatite  as  interstitial  material  (Figs.  2  and  3,  PL  II).  This 
texture  is  called  granulitic  (Fr.  granulite,  two-mica-granite)  or  aplitic.  It  has  been, 
incorrectly  named  panidiomorphic  (GT.irav,  all),  but  such  a  texture  is  inconceivable 
in  a  compact,  crystalline  rock.  If  micropegmatite  predominates  as  interstitial  mater- 
ial, the  texture  is  micropegmatitic. 

The  above  characteristic  sequence  of  separation  gradually  disappears  in  the  rocks 
from  diorite  to  those  that  are  more  basic.  Thus  in  the  gabbroic  texture  (PL  II,  Fig.  5) 
the  constituents  are  equally  developed,  showing  that  they  crystallized  nearly  simul- 
taneously. This  allotriomorphic1  (Gr.  aXXorpio  y,  foreign)  texture  gives  place,  in  rocks 
containing  still  more  of  the  basic  constituents,  to  the  ophitic  texture  (PL  II,  Fig.  6) 
of  traps  and  diabases,  in  which  the  colored  minerals  form  the  mesostasis.  With  a 
still  further  decrease  in  the  amount  of  feldspar,  this  texture  also  is  lost,  and  the 
feldspar-poor  picrites,  and  especially  the  feldspar-free  peridotites,  again  have  an  allo- 
triomorphic-granular  texture.  The  intergrowths  of  pyroxene  and  olivine  in  the 
peridotites,  in  some  cases,  show  a  poikilitic  (Gr.  TrowctXos,  spotted)  or  an  implication 
texture  (Lat.  implicare,  to  fold  in),  presumably  representing  a  eutectic  mixture.- 

I  3L          Jl  tt  Y 


I 


FIG.  129. — Sequence  of  crystallization  in  granite. 

If  the  physical  conditions  were  altered  during  any  stage  of  the  crystallization  of 
a  rock,  a  hiatus  occurred  in  the  separation.  This  is  characteristically  seen  in  extru- 
sive rocks  in  which  larger  intratelluric  (Lat.  intra,  within,  tellus,  the  earth)  pheno- 
crysts  (Gr.  <paivu  show)  stand  out  from  a  finer  groundmass  and  produce  the  porphy- 
ritic  texture  (PI.  Ill,  Fig.  1).  When  these  rocks  have  a  completely  crystalline  ground- 
mass  they  are  also  called  holocrystalline-porphyritic  (Gr.  oXos,  entirely),  in  contrast 
to  hypocrystalline-porphyritic,  in  which  a  glassy  basis  is  present. 

There  is  great  variety  in  the  textures  of  groundmasses,  especially  in  silicic  rocks. 
The  groundmass  in  Fig.  1,  PI.  Ill  is  micrograniiic,  that  in  Fig.  2  is  glassy  or  vitrophyric 
(Lat.  vitrum,  glass)  and  has  a  fluidal  eutaxitic  (Gr.  eura^ta,  good  order)  development. 
Fig.  3  shows  the  spherulitic  (Gr.  o-^mpor,  sphere)  or  granophyric  (Lat.  granum,  grain) 
texture  of  quartz-porphyry.  Between  these  textures  there  are  all  possible  transitions. 
In  many  of  them  the  crystalline  condition  cannot  be  proved  with  certainty,  and  such 
rocks  are  called  felsophyric  or  microfelsitic  quartz-porphyries. 

A  fluidal  arrangement  of  small  feldspar  laths  is  characteristic  for  the  groundmass 
of  the  trachylic  texture  (PI.  Ill,  Fig.  4).  Where  feldspar  individuals  of  trachyte  are 

lXenomorphic,  ROHRBACH  (1886),  allotrimorphic,  ROSENBUSCH  (1887). 


JOINTING  AND  TEXTURES  199 

more  equidimensional  in  outline  and  less  uniformly  distributed,  the  texture  is  ortho- 
phyric.  In  some  andesites  there  is  a  more  or  less  fluidal  arrangement  of  the  plagioclase 
laths  of  the  groundmass;  this  is  the  pilotaxitic  (Gr.  xiXo  s,  felt,  rd£is,  order)  texture.  In 
others  these  laths  are  more  irregular  in  their  distribution,  as  in  the  andesitic  or  hyalopi- 
lilic  (Gr.  CoXo  s,  glass)  texture  shown  in  Fig.  5,  PL  III. 

Among  the  basic  melaphyres  there  occurs  a  texture  analogous  to  the  ophitic, 
namely  the  intersertal  (Fig.  6,  PL  III).  In  this  texture  the  interstices  between  the 
feldspar  laths  are  filled  with  glass.  In  rocks  of  the  basalt  group,  which  are  still 
poorer  in  feldspar,  the  constituents  of  the  groundmass  form  a  felty  aggregate  of  mi- 
crolites  which  can  hardly  be  resolved  by  the  microscope.  This  is  the  microlitic  texture. 
In  the  last  two  cases  the  true  porphyritic  character,  that  is  the  occurrence  of  two  gen- 
erations of  the  same  mineral  separated  by  a  time  interval,  may  be  inconspicuous. 

Internal  Textures  of  Contact-rocks  and  Crystalline  Schists.— 
Several  very  different  textures  occur  among  the  rocks  which  are 
included  under  the  name  of  crystalline  schists.  Schlieren  granites, 
diorites,  or  gabbros  generally  have  the  same  textures  as  the  corre- 
sponding normal  rocks,  but  in  another  group,  mechanical  textures 
predominate  and  in  many  cases  nearly  conceal  the  original  char- 
acter. The  textures  of  the  metamorphic  schists  differ  entirely 
from  those  found  in  igneous  rocks.  They  were  long  ago  recognized 
as  being  the  same  as  those  characteristic  at  contacts,  and  this 
identity  of  textures  shows  that  the  physico-chemical  processes 
which  were  active  during  their  formation  were  also  identical. 

A  microscopic  study  of  the  textures  of  metamorphic  schists,  and 
contact  and  igneous  rocks  shows  that  only  in  the  latter  is  there  a 
characteristic  sequence  of  separation  of  the  minerals.  Although 
individual  constituents  may  stand  out  from  the  groundmass  on 
account  of  greater  size  or  more  perfect  crystal  form,  careful  study 
shows  that  all  of  the  minerals  of  the  apparent  groundmass  occur 
also  as  inclusions  in  the  larger  crystals.  The  latter,  therefore, 
cannot  be  older.  Such  textures  are  called  pseudoporphyritic.  In 
the  knoten  texture  the  larger  crystals  appear  megascopically  in  the 
form  of  knots  upon  fracture-planes,  in  the  garben  texture  they 
occur  in  sheaf-like  forms. 

If  the  peculiarities  of  the  textures  of  contact-rocks  and  crystalline  schists  be  examined 
carefully,  numerous  details  will  be  found  which  show  that  the  molecules  were  much  less 
mobile  during  the  crystallization  of  these  rocks  than  they  were  hi  igneous  magmas. 
But  at  the  same  time  it  is  certain  that  the  recrystallization  did  not  take  place  when  the 
rocks  were  hi  a  solid  state,  as  believed  by  Rosenbusch. 

In  the  more  important  typical  occurrences  of  contact-rocks  and  metamorphic 
schists,  the  materials  for  the  different  constituents  clearly  came  from  rather  distant 
sources.  The  condition  of  the  rock  substance,  however,  was  too  viscous  to  permit 
the  formation  of  pure  crystals,  consequently  there  are  usually  many  inclusions, 
especially  in  the  larger  phenocrysts.  This  gives  rise  to  the  sieve  texture  (PL  IV, 


200         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

Fig.  1).  In  many  cases  intimately  intergrown  aggregates  of  different  minerals  occur 
instead  of  simple  homogeneous  crystals,  and  these  may  be  so  dense  that  they  are  not 
transparent  except  in  very  thin  sections  (Fig.  2,  PL  IV). 

A  further  proof  of  the  slight  mobility  of  the  molecules  is  the  fact  that  remnants 
of  the  original  texture  are  preserved  in  places  (palimpsest  texture),  and  may  even 
appear  distinctly  to  the  unaided  eye,  as  in  the  porphyritic  (Fig.  83)  and  ophitic  tex- 
tures of  innumerable  greenstone-schists.  Well-preserved  fossils  (Fig.  130)  in  recrys- 
tallized  sediments  may  also  show  that  there  has  been  but  slight  movement.  In  other 
cases  the  original  texture  cannot  be  seen  without  the  microscope,  as  in  the  helizitic  tex- 
ture (Gr.  e\i£,  spiral:  PL  IV,  Fig.  3),  in  which  bands  and  strings  of  inclusions,  represent- 
ing the  original  schistosity  of  the  rock,  cut  across  the  newly  formed  constituents 
of  the  altered  rock. 

In  the  mosaic  texture  (PL  IV,  Fig.  4),  which  likewise  is  very  characteristic  of  these 
rocks,  the  individual  grains  lie  adjacent  to  each  other  like  irregular  paving-blocks, 
and  meet  with  rather  straight  contacts.  Similar  textures  are  shown  in  Fig.  1,  PL  V, 
and  Fig.  1,  PL  VI.  In  Fig.  2,  PL  V,  and  Fig.  2,  PL  VI,  the  meeting-lines  between  the 
grains  are  very  irregular  or  toothed.  This  sutured  texture,  in  many  cases,  occurs  in 


FIG.  130. — Crinoid  stem  in  coarse  marble.      FIG.  131. — Quartz-phyllite.   Sunk,  Steier- 
Bayumkol  Valley,  Tien-Schan.  mark.     Shows  folded  sericite  films. 

association  with  the  mosaic  texture.  It  is  especially  striking  in  itacolumite  (PL  V, 
Fig.  3)  in  which  the  irregular  quartz  grains  are  not  cemented  and  the  rock  is  flexible. 

All  the  micaceous  minerals  of  rocks  which  recrystallized  under  high  pressure 
occur  in  parallel  planes  (Fig.  131).  This  is  the  crystallization-schistosity  of  Becke. 
Wherever  a  larger  mica  crystal  lies  across  the  direction  of  these  planes,  that  is  with 
its  base  parallel  to  the  direction  of  the  pressure,  it  shows  unusually  poor  outlines  and 
a  thick-tabular  development  (PL  IV,  Fig.  5). 

Becke  gave  innumerable  new  names  to  the  textures  of  the  metamorphic  schists 
in  an  attempt  to  separate  them  from  those  of  contact-rocks.  He  compared  the  crys- 
tallization from  a-  viscous  mass  to  sprouting,  and  called  all  rocks  which  originated  in 
this  manner  crystalloblastic  (Gr.  /SXaoros  sprout).  He  distinguished  further:  grano- 
blastic  or  cyclopic  textures,  corresponding  to  the  mosaic  textures  of  igneous  rocks, 
lepidoblastic  corresponding  to  scaly,  nematoblastic  =  fibrous,  homooblastic  =  equi-granu- 
lar,  porphyroblastic  =  pseudo-porphyritic,  diablastic  =  sieve  texture,  poikiloblastic  = 
helizitic,  idioblastic  =  automorphic,  xenoblastic  =  xenomorphic.  Blasto  granitic  was 
applied  to  rocks  having  a  palimpsest  texture  with  an  underlying  veiled-granitic 


JOINTING  AND  TEXTURES 


201 


texture,  and  in  an  analogous  manner  blastophitic,  blastoporphyritic,  blastopsephitic, 
blastopsammitic,  etc.,  were  used.  The  mortar  or  murbruk  texture,  which  originated 
in  brecciation,  was  called  porphyrodastic. 

Internal  Textures  of  Sedimentary  Rocks. — Three  different  kinds 
of  constituents,  clastic,  organic,  and  authigenic,  influence  the  tex- 
tures of  sedimentary  rocks.  The  clastic  constituents  are  most 
characteristic,  and  the  term  petite,  psammite,  or  psephite  indicate 
the  size  of  the  grains  of  which  they  are  composed. 

Besides  minute  quartz  splinters  and  occasional  fragments  of 
purely  accidental  constituents,  there  is  usually  little  to  see  in 
typical  pelites,  even  under  strong 
magnification.  On  the  other  hand, 
the  clastic  nature  of  the  psammites 
is  very  distinct.  The  seolian  or  al- 
luvial origin  of  many  of  them  may 
be  shown  by  the  fact  that  the  chief 
constituent,  quartz,  occurs  in 
rounded  grains  in  the  former  (PL 
V,  Fig.  6)  and  in  angular  frag- 
ments in  the  latter  (PL  V,  Fig.  5). 
The  cement  also  in  many  cases 
occurs  in  very  characteristic  forms, 
even  though  partially  modified  by 
diagenic  processes.  For  example, 
the  water-clear  quartz  cement  in 

the  rock  shown,  in  PL  V,  Fig.  6  is  a  secondary  growth  around 
the  originally  rounded,  dark-bordered  sand  grains. 

The  second  class  of  sediments,  such  as  crinoid  limestones  (Fig. 
132),  owe  their  textures  to  the  preserved  skeletal  parts  of  organ- 
isms. As  was  shown  above,  different  kinds  of  skeletons  possess 
different  degrees  of  durability;  siliceous  skeletons  being  least,  and 
bones  most  durable.  Calcareous  fossils  may  be  easily  separated 
into  two  groups.  One  of  these  is  based  upon  the  character  of  the 
inorganic  material  of  the  skeletons,  whether  calcite  or  conchite. 
Original  conchite  is  never  preserved,  for  it  is  altered  by  diagenic 
processes  to  granular  calcite.  On  the  other  hand,  many  calcite 
fossils  preserve  their  structures,  especially  if  they  were  originally 
coarsely  crystalline,  as  are  the  skeletons  of  echinoderms,  while 
fine-fibrous,  ray-like  foraminifera  rarely  preserve  it.  The  second 
group  of  microscopic  structures  of  calcareous  skeletons  depends 


FIG.  132.— Cri 
grating  texture. 
Algau. 


oid  limestone  with 
Vilstal,  Pfronten, 


202         FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 

upon  the  distribution  of  the  original  organic  material  in  them. 
The  structure  is  distinctly  brought  out  by  the  diagenic  altera- 
tion of  this  material  to  carbonaceous  matter,  for  example  in  the 
grating  structure  of  crinoids  (Fig.  132). 

The  third  group  of  constituents  which  may  influence  the 
structure  of  sediments  differs  from  the  other  two  in  being  authi- 
genic.  These  constituents  originated  either  by  direct  chemical 
sedimentation,  as  in  gypsum  and  rock-salt  deposits,  or  by  diagenic 
processes.  In  the  former  case  they  may  be  quite  coarsely  crystal- 
line, in  the  latter,  especially  in  carbonate-  and  silicate-rocks,  they 
may  consist  of  fine-grained  crystalline  aggregates.  Even  when 
the  constituents  are  nearly  submicroscopic  in  size,  they  may  be 
distinctly  recognized  as  authigehic. 

Finally,  the  oolitic  texture  is  typical  in  many  carbonate-rocks. 
In  these  the  original  radial-concentric  development  may  be  dis- 
tinctly preserved  (Fig.  5,  PL  VI),  but  more  commonly  a  diagenic 
recrystallization  has  produced  a  granular  aggregate  of  calcite 
(Fig.  6,  PL  VI). 

It  may  be  pointed  out  in  conclusion  that  secondary  fillings  in 
fissures  and  cracks,  or  infiltrations  along  bedding-  and  schistosity- 
planes,  may  produce  confusion  in  the  microscopic  examination  of 
the  textures  of  sedimentary  rocks.  In  many  cases  these  can  be 
recognized  in  the  thin  section  only  by  their  much  more  distinct 
crystalline  development. 

Mechanical  Textures. — All  rock-textures  modified  by  the  action 
of  orogenic  processes  are  included  under  the  term  mechanical 
textures.  They  may  be  seen  megascopically  in  many  cases,  most 
distinctly  in  folded  bedded-rocks  and  in  transversely-schistose 
argillites.  The  degree  of  consolidation  of  sediments,  and  the 
numerous  fissures,  veins,  and  pressure-sutures  of  carbonate-jocks, 
all  indicate  mechanical  action.  Mechanical  alterations  may  be 
observed  even  with  the  unaided  eye  in  true  crystalline  rocks,  for 
many  coarse-grained  marbles  are  mashed  to  wax-like,  dense 
aggregates,  and  the  crushed  quartz  of  granitic  rocks  takes  on  a 
sand-like  appearance. 

The  action  of  mechanical  forces  appears  much  more  variable  and 
distinct  in  the  internal  structures  of  rocks  than  in  the  external. 
In  limestones  the  most  common  form  of  alteration  is  the  destruction 
of  the  organic  texture,  but  there  are  many  other  kinds.  Various 
minerals  behave  very  differently  toward  pressure.  For  example, 


JOINTING  AND  TEXTURES 


203 


mica  may  be  greatly  bent  without  being  broken  (PL  IV,  Fig.  6), 
while  quartz  shows  distinctly  the  slightest  trace  of  such  action  by  its 
undulatory  extinction  in  polarized  light.  With  further  fragmenta- 
tion the  quartz  is  cut  by  transverse  and  longitudinal  cracks  which 
are  filled  with  grains  produced  by  the  mechanical  movement,  and 
between  these  cracks  coarse  remnants  of  the  original  crystals 
remain.  This  is  the  mortar  or  murbruk  texture  (PL  V,  Fig.  4). 
Olivine  acts  in  much  the  same  way. 

The  amount  of  bending  or  breaking  of  certain  minerals  serves  as  a  scale  for  the 
measurement  of  the  degree  of  mechanical  action  which  the  rocks  have  suffered.  In- 
termediate between  the  flexible  mica  and  the  brittle  quartz  stand  the  other  minerals. 
Diopside  and  hornblende  may  still  be  bent,  although  the  latter  may  be  somewhat 
displaced  along  cleavage  lines,  so  that  a  cross-section  resembles  parquetry  (Fig.  133). 
If  these  textural  features  were  produced  by  mechanical  processes  which  were  active 
during  the  solidification  of  the  rock  they  are 
called  protodastic  (Gr.  irptaro  s,  first,  /cXdco,  to 
break),  if  they  originated  after  the  rock 
solidified  they  are  called  cataclastic  (Gr.  KOTO., 
after). 

Calcite  is  of  especial  interest,  not  only  on 
account  of  its  wide  distribution  but  also  be- 
cause such  thorough  experiments  have  been 
made  upon  it.  It  has  been  found  that  at 
ordinary  temperatures  calcite  quite  readily 
becomes  cataclastic  (PI.  VI,  Fig.  3),  and  then 
resembles  mashed  quartz.  At  a  tempera- 
ture of  several  hundred  degrees,  however, 
it  has  considerable  plasticity,  due  partially 
to  gliding  and  the  development  of  fin0  twin- 
ning-lamellae,  partially  to  direct  bending  of 
the  lamellae  themselves.  In  numerous 
granular  limestones  a  re-formation  has  taken 

place  along  twinning-lamellse  of  the  calcite  but  the  mineral  itself  shows  no  signs  of 
fracturing  (PI.  VI,  Fig.  4).  Such  rocks,  therefore,  may  be  assumed  to  have  been 
under  the  action  of  mechanical  forces  in  the  presence  of  heat. 

All  of  the  above  characters  are  due  to  lateral  pressure  acting  under  an  enormous 
overlying  load.  With  a  sudden  cessation  of  the  stress,  however,  as  by  faulting,  such 
minerals  as  calcite,  mica,  etc.,  which  are  highly  plastic  under  pressure,  will  be  completely 
fractured.  Under  such  conditions,  on  account  of  the  friction  in  the  fault-clefts  or 
slipping-planes,  the  whole  rock  may  be  uniformly  reduced  to  a  grit-breccia,  or  there 
may  be  formed  a  fine,  porous  material  containing  more  or  less  abraded  or  striated 
blocks  of  the  country-rock  resembling  glacial  boulders,  or  compact  rocks  may  be  pro- 
duced with  pronounced  flaser  structures  and  showing  fragments  of  the  original  rock 
as 'eyes  in  a  brecciated  schistose  groundmass.  In  addition,  many  new  minerals,  chiefly 
mica-like,  are  formed  by  circulating  juvenile  solutions.  In  many  cases  the  breaking 
up  has  gone  so  far  that  only  fine  dust  remains,  as  in  the  clay-schist  dikes  of  the  Harz, 
or  there  is  formed  a  uniform,  dense,  schistose  rock  like  the  Pfahl  schist,  whose  origin 
from  granite  would  never  be  suspected  from  its  present  appearance.  Intense  crushing 
also  occurs  at  the  base  of  great  overthrust  faults,  and  materials  from  the  underlying 


FIG.    133. — Crushed  actinolite  in  ser- 
pentine.    Stubachtal,  Salzburg. 


204 


FUNDAMENTAL  PRINCIPLES  OF  PETROLOGY 


rocks,  planed  off  by  the  movement,  are  mingled  and  kneaded  together,  and  finally 
consolidated  as  dense  rocks  with  a  peculiar  kneaded  texture  (Fig.  103).  All  of  these 
rocks  produced  by  brecciation  are  grouped  together  as  mylonites  (Gr.  nv\rj,  mill). 
Inclusions,  Concretions,  and  Secretions. — There  still  remain  to  be  mentioned 
inclusions,  concretions,  and  secretions.  Inclusions  embrace,  primarily,  the  foreign 


FIG.  134. — Contact-breccia.     Gabbro-diorite  intruded  by  granite.      Schonberg  along 
the  Bergstrasse.     (Prof.  Dr.  Klemm,  photo.) 

rock-  or  mineral-fragments  which  are  contained  in  igneous  rocks.  In  some  cases  they 
have  preserved  their  original  fragmental  form,  in  others  they  have  been  partially 
fused  or  metamorphosed  by  the  action  of  the  molten  magma  or  even  injected  with  the 


FIG.  135. — Loess  kindl.  Utten- 
hofen,  Rheinpfalz.  The  lower  ex- 
ample is  very  sandy. 


FIG.  136. — Crystallized  sandstone, 
tainebleau  near  Paris. 


Fon- 


igneous  material.     When  the  inclusions  are  endogenic  (Gr.  evdov,  within),  as  in  many 
lamprophyres,  they  usually  have  sharply  denned  borders  and  represent  fragments  of 


JOINTING  AND  TEXTURES  205 

material  which  solidified  in  the  magma  itself  in  the  bowels  of  the  earth.  The  most 
important  of  such  inclusions  are  the  olivine  nodules  so  common  in  basalts. 

A  second  group  of  inclusions  are  exogenic  (Gr.  e£a>,  out),  and  consist  of  fragments 
of  the  country-rock,  torn  loose  by  the  igneous  magma.  In  many  plutonic  rocks  these 
have  been  so  far  assimilated  that  they  stand  out  from  the  otherwise  homogeneous 
rocks  only  as  poorly  defined  spots,  the  so-called  basic  inclusions.  Gneiss  inclusions 
in  granite  may  have  preserved  their  original  schistose  character  and  have  become 
filled  with  injections  of  the  igneous  rock.  Inclusions  are  especially  numerous  in 
small  dikes  and  in  the  border-zones  of  large  intusives  where  they  form  contact- 
breccias  (Fig.  134). 

A  great  variety  of  exogenic  inclusions  occur  in  volcanic  tuffs.  They  consist  of 
normal  contact-rocks,  which  were  clearly  altered  before  the  magma  reached  the  sur- 
face, and  fragments  of  the  country-rocks  through  which  the  rising  magma  passed. 
The  latter  are  usually  fritted.  Especially  interesting  to  the  mineral  collector  are  the 


FIG.  137. — Agate,  showing  conduit.     Oberstein  a.  N. 

inclusions  and  ejectamenta  of  sodic  rocks,  which  in  many  cases  are  entirely  saturated 
by  the  constituents  of  the  magma. 

Concretions  (Lat.  concretus,  grown  together)  are  concentrations  of  certain  constitu- 
ents in  sediments.  In  some  cases  they  originated  during  the  rock  formation,  in  others 
by  later  processes.  Here  belong  the  clay-pockets  of  sandstones,  the  much-fissured, 
lens-like  septaria  of  clays,  nests  of  gypsum,  pyrite,  marcasite,  and  siderite  hi  marls 
and  argillites,  of  flint  in  chalk,  and  of  menilite  in  the  siliceous  schists;  also  loess-kindl 
(Fig.  135)  representing  calcareous  concretions,  sand-filled  -calcite-clusters  in  the  so- 
called  crystallized  sandstone  (Fig.  136),  knots  and  bands  of  hornstone  and  carnelian 
in  limestones  of  different  formations,  and  finally  masses  of  limonite,  phosphorite, 
celestite,  etc. 

Secretions  (Lat.  secretus,  separated),  or  more  correctly  infiltrations,  include  all 
cavity-fillings  in  rocks.  Among  these  are  amygdules  and  geodes  of  calcite,  zeolites, 
and  agate  (Fig.  137)  in  vesicules  of  igneous  rocks,  veins  of  quartz  and  calcite  in  all 
kinds  of  rocks,  and  all  mineral  aggregates  in  cavities  of  any  kind.  The  material  in 
all  cases  was  brought  in  from  external  sources. 


^  '"T^.*  V  • 

.^t. 


PLATE  II. 


H€-';  IBs 
^•$ji.-*->3& 


FIG.  1. — Granitic  texture.  (After 
Berwerth.)  The  dark  constituents  show 
distinct  boundaries  against  the  light. 
Quartz  (the  lightest  mineral  in  the  figure) 
was  the  last  mineral  to  crystallize.  It  fills 
the  interstices  between  the  earlier  con- 
stituents. Ordinary  light. 


FIG.  2. — Granulitic  texture.  Quartz 
shows  distinct  boundaries  against  the  re- 
maining constituents.  Polarized  light. 


FIG.  3. — Micropegmatitic  texture. 
The  eutectic  mixture  of  orthoclase  and 
quartz  in  graphic  intergrowth  was  the  last 
product  of  crystallization.  Polarized  light. 


FIG.  4. — Monzpnite  texture.  The  or- 
thoclase fills  the  interstices  between  the 
plagioclase  laths.  Polarized  light. 


FIG.  5. — Gabbro  texture.     Irregular, 
granular  arrangement.     Polarized  light. 


FIG.  6. — Ophitic  texture.  Large  au- 
gite  individuals  fill  the  spaces  between 
plagioclase  laths.  Ordinary  light. 


PLATE  III. 


FIG.  l.—Porphyritic  texture.     (After  FIG.  2. — En  taxi  tic    texture.     The 

Berwerth.)     The  groundmass  is  micro-  groundmass  shows  distinct  fluidal  texture, 

granitic,  and  in  this  lie  large  phenoerysts  Ordinary  light, 
of  quartz  and  feldspar.     Polarized  light. 


FIG.  3.— Spherulitic  texture.  The 
groundmass  consists  predominantly  of 
radially  arranged  quartz-feldspar  aggre- 
gates. Polarized  light. 


FIG.  4. — Trachytic  texture.  (After 
Berwerth.)  Feldspar  microlites  are  ar» 
ranged  in  flow-lines  around  the  pheno- 
crysts. Polarized  light. 


FIG.  5. — Hyalopilitic  texture.  The 
groundmass  is  rich  in  glass^and  contains 
unoriented  microlites  of  plagioclase  and 
augite.  Ordinary  light. 


.••/A 


FIG.  6. — Intersertal  texture.  (After 
Berwerth.)  The  last  remnant  of  the 
magma  solidified  as  a  microlite-filled  glass. 
It  fills  the  interstices  between  the  remain- 
ing constituents.  Ordinary  light. 


PLATE  IV. 


FIG.  1. — Sieve  texture.  A  garnet 
crystal  with  poor  boundaries  is  filled  with 
inclusions  of  the  other  constituents.  Or- 
dinary light. 


ggre- 
gate of  pyroxene  and  plagioclase.  Polar- 
ized light. 


FIG.  3. — Helizitic  texture.  Biotite, 
sillimanite,  and  ilmenite  crystals,  ar- 
ranged parallel  to  the  original  bedding, 
intersect  a  crystal  of  cordierite  which 
takes  up  the  greater  part  of  the  field  of 
view.  Polarized  light. 


FIG.  4. — Mosaic  texture.  The  albite 
crystals  are  arranged  as  in  a  mosaic. 
Polarized  light. 


FIG.  5. — Biolite  with  its  long  direc- 
tion at  right  angles  to  the  cleavage  of  the 
rock.  Ordinary  light. 


FIG.  6. — Bent  biotite  lamellae.  Though 
the  mica  is  greatly  bent,  it  shows  no 
breaks.  Polarized  light. 


PLATE  V. 


FIG.  1.  — Mosaic  texture.  Quartz 
grains  with  rather  plane  contacts.  Polar- 
ized light. 


FIG.  2. — Sutured  texture.  The  quartz 
individuals  interlock  and  are  firmly 
united.  Polarized  light. 


FIG.  3. — Sutured  texture  in  itacolu- 
mite.  The  quartz  individuals  interlock 
but  are  not  firmly  united .  Ordinary  light. 


FIG.  4. — Mortar  or  murbruk  texture. 
Shows  fine  quartz  grains  in  fissures  in 
quartz  grains  with  undulatory  extinction. 
Polarized  light. 


FIG.  5. — Clastic  texture.  Angular 
quartz  "grains,  clearly  alluvial,  in  a  dense, 
clay-like  cement.  Polarized  light. 


FIG.  6. — Clastic  texture.  Rounded 
quartz  grains,  clearly  aeolian,  with  rims 
of  secondary  quartz.  Ordinary  light. 


PLATE  VI. 


FIG.  1. — Mosaic  texture.  Calcite 
crystals  with  rather  plane  contacts. 
Polarized  light. 


FIG.  2. — Sutured   texture.     The  calcite 
grains  interlock.     Polarized  light. 


FIG.  3. — Cataclastic  texture.  Fine 
calcite  aggregates  between  strained  rem- 
nant* of  larger  grains  of  calcite.  Polar- 
ized light. 


FIG.  4. — Mechanical  texture.  The 
twinning  lamellae  of  the  calcite  are  bent 
yet  the  mineral  is  not  broken.  Ordinary 
light. 


FIG.  5. — Oolitic  texture.  Show?  the 
original  radial  and  concentric  arrange- 
ment within  the  individual  spherules. 
Polarized  light. 


FIG.  6. — Oolitic  texture.  The  spher- 
ules have  been  altered  by  diageresis  to  a 
granular  aggregate.  Polarized  light. 


INDEX 


Abrasion,  89 

Abyssal  differentiation,  48 

rocks,  17 

Accessory  minerals,  31 
Acid  rocks,  33 
Adiagnostic  rocks,  190 
Adinole,  137 

Aeolian  sediments,  95,  97 
Agate,  145,  205 
Age  classification  of  igneous  rocks,  28. 

of  igneous  rocks,  27 

of  Catastrophies,  20 
Agents  of  contact-metamorphism,  116 
Allothigenic  constituents,  6 
Allotriomorphic,  198 
Alluvial  sediments,  95,  99 
Alluvium,  85 

Alpine  granite,  anomalous  character,  59 
Alps,  origin  of  border  zone,  135 
Alunitization,  154 
Amygdaloids,  144,  191 
Amygdule,  fillings,  145 
Amygdules,  205 
Anamorphism,  78 
Andernach,  44 
Andesitic  texture,  199 
Anogenic  metamorphism,  164 

rocks,  7 

Aphanitic  rocks,  190 
Aplite  related  to  granite,  22 
Aplites,  5,  49 

never    occur    in    unmetamorphosed 

sediments,  23 
Aplitic  injections,  121 

texture,  198 
Apophyses,  28 
Aragonite  formation,  103 
Argillites,  96 

contact-metamorphism  of,  123 
Aschaffenburg,  52 
Aschistic  rocks,  49 
Ashes,  volcanic,  25 
Atmosphere,  6 


Augen-gneiss,  56 

Authigenic  constituents,  6 

Automorphic,  197 

Average  composition  of  earth,  34 

B 

Bad-land  topography,  89 
Banded  hornfels,  63 

-gneisses,  66 

rocks,  41,  191 

Basic  igneous  rocks,  contact-metamorph- 
ism of,  131 

rocks,  33 
Basis,  17 
Bauxite,  79,  83 
Bavarian  Forest,  88 
Bedded  rocks,  7,  194 
Bedding,  cross,  98 

diagonal,  98 

direct,  101 

indirect,  101 
Bennan  Head,  Arran,  67 
Bioliths,  1.06 

Bitumen,  formation  of,  110 
Bituminous  coal,  109 
Black  Hills,  S.  D.,  88 
Blastogranitic  texture,  200 
Blastophitic  texture,  201 
Blastoporphyritic  texture,  201 
Blastopsammitic  texture,  201 
Blastopsephitic  texture,  201 
Block-lava,  26 
Blue-mud,  101 
Bombs,  25 
Borax-seas,  80 
Bozen,  South  Tyrol,  44 
Breccia,  endogenic,  189 
Brogger's  diagrams,  71 
Brohl  valley,  44 
Brown  coal,  109 
Bunsen's  theory  of  differentiation,  54 


Calcareous  sinter,  formation  of,  104 
Calciphyres,  128 


207 


208 


INDEX 


Calcite  formation,  103 

Carbonaceous  deposits,  108 

Carbonate  rocks,  contact-metamorphism 

of,  127 

Carbon  dioxide  from  volcanic  eruptions, 84 
Cataclastic  texture,  203 
Catamorphism,  78 
Catogenic  rocks,  7    * 
Caustobioliths,  106 
Cave  formation,  75 
Cavernous  rocks,  191 
Cellular  rocks,  191 
Center  of  earth,  condition  of,  8 
Chelif  River,  Algeria,  80 
Chemical  composition  of  igneous  rocks,  34 

sediments,  95 

weathering,  74 

formerly,  83 
Chief  minerals,  31 
Chilian  pampas,  80 
Chlorite-schists,  132 

Christiania  basin,    Norway,    differentia- 
tion in,  54 

Classification  of  crystalline  schists,  162 
Clastic  rocks,  6 
Clay,  77 

Clay-pockets,  205 
Clay-slate  needles,  96 
Climatic  zones  of  weathering,  82 
Coal,  108 

method  of  formation,  109,  113 
Coarse-grained  rocks,  190 
Columnar  jointing,  186 
Comagmatic  region,  19 
Complementary  dikes,  48 
Composite  dikes,  53 
Composition  of  igneous  rocks,  31 

of  volcanic  emanations,  15 
Compound  rocks,  5 
Concretions,  204,  205 
Conglomerates,  96 
Consanguinity,  53 
Constitution  schlieren,  44 
Contact-limestone  minerals,  136 
Contact-metamorphic  agents,  116 
Contact-metamorphism,  116 

by  extrusive  rocks,  137 

by  plutonites,  119 

of  argillites,  123 

of  basic  igneous  rocks,  131 

of  carbonate-rocks,  127 

susceptibility  of  a  rock  to,  121,  124 


Coral  reefs,  106 
Corrasion,  89 

Cross,   Iddings,   Pirsson,   and  Washing- 
ton's system,  72 
Cross-bedding,  98 
Cryptogenic  rocks,  7 
Cryptomerous  rocks,  190 
Cryptovolcanic  activity,  27  . 
Crystalline  rocks,  5 
schists,  7,  157 

action  of  granite  on,  176 

characteristics  of,  11 

classification  of,  162 

early  ideas  regarding,  157 

facies  of,  173 

origin  of,  179 

thickness  of,  177 

variability  of,  160 

younger,  12,  158 
Crystallization-schistosity,  200 
Crystallization  sequence,  197 
Crystallized  sandstone,  205 
Crystalloblastic  texture,  200 
Crystalloids,  77 
Cyclopic  texture,  200 


Deep-seated  rocks,  17,  24 
Deflation,  89 
Dense  rocks,  190 
Denudation,  88 
Desmosite,  138 
Devil's  wall,  Bohemia,  93 
Diabase-hornfels  minerals,  136 
Diablastic  texture,  200 
Diagenesis,  74,  111,  161 
Diagonal-bedding,  98 
Diaschistic  rocks,  49 
Diatremes,  21,  27 
Differentiation,  abyssal,  48 

dikes,  49 

laccolithic,  48 

magmatic,  44 

schlieren,  44 

theories,  54 
Dike  rocks,  49 
Dikes,  24 

composite,  53 

double,  53 

on  the  Isle  of  Arran,  93 

welded,  52 


INDEX 


209 


Direct  bedding,  101 
Dislocation  metamorphism,  164 
Dolinas,  75 
Dolomite-ash,  130 
Dolomitization,  113 
Double  dikes,  53 
Dreikanter,  89,  98 

Dynamometamorphism,  56,  164,  169 
in  Alps,  135 

E 

Earth  pillars,  102 

Earth's  crust,  formation  of,  9 

Eclogite,  132 

Effusive  rocks,  17,  25 

Elements,  distribution  of,  33 

Endogenic  breccias,  189 

contact  phenomena,  47 

inclusions,  204 
Eozoon,  12,  159 
Epi-rocks,  174 
Erosion,  89 
Eruptions,  explosive,  27 

quiet,  27 

Erzgebirge,  48,  61 
Essential  minerals,  31 
Eutaxitic  groundmass,  198 
Eutectic  intergrowth  of  calcite  and  dolo- 
mite, 159 

mixtures,  38 
Exaration,  89 
Exogenic  inclusions,  205 
Explosive  eruptions,  27 
Explosiveness  of  magma,  22 
Extrusive  rocks,  17,  25 

contact-metamorphism  of,  137 


Facies  of  crystalline  schists,  173 

of  granite,  45 
Felsophyric,  198 
Fichtelgebirge,  63,  86,  87 
Fine-grained  rocks,  190 
Fission  algae,  113 
Fleckschiefer,  126 
Fluctuation  texture,  191 
Fluidal  texture,  191 
Fluviatile  sediments,  99 
Folding,  fractureless,  165 
Forms  of  weathering,  85 
Fossil  sediments,  114 
Fractureless  folding,  165 


Fragmental  rocks,  6 

Frankland,  81 

Fresh-water  limestones,  formation  of,  104 

Friction-breccias,  170 

Fritted  rocks,  137,  189 

Frothy  rocks,  191 

Fruchtschiefer,  126 

G 

Gabbroic  texture,  198 
Ganggefolgschaft,  49 
Garbenschiefer,  126 
Garben  texture,  199 
Gases  from  volcanoes,  84 
Gels,  6,  77 
Geodes,  205 

Geologic  pipe  organs,  75 
Giant-grained  rocks,  190 
Glacial  sediments,  95 
Glashiittental,  Schemnitz,  42 
Glassy  groundmass,  198 

rocks,  6 

Glauconite-sand,  101 
Gossan,  142 
Grain,  87,  184 
Granitic  texture,  39,  197 
Granoblastic  texture,  200 
Granoblasts,  176 
Granophyric  groundmass,  198 
Granular  rocks,  17,  197 
Granularity  of  dikes,  48 
Granulitic  texture,  39,  198 
Graphical    representations    of    composi- 
tion, 68 

Graphite  deposits,  147 
Gravel,  96 
Green-mud,  101 
Greensand,  101 

Greenstones,  formation  of,  151 
Greenstone-schist,  131 
Greisen,  146 
Grus,  80 
Grush,  80 
Gumbel's  age  classification,  28 

theory  of  diagenesis,  161 
Gypsum  chimneys,  75 

formation  of,  104 

H 

Haphazard  texture,  46,  191 
Hawaiian  lava  temperature,  39 
Hebrides,  42 


210 


INDEX 


Helm's  theory,  169 

Helizitic  texture,  118,  200 

Henry  Mountains,  Utah,  64 

Herculaneum,  21 

Holocrystalline  rocks,  197 

Holocrystalline-porphyritic  texture , 
198 

Homogeneous  volcanoes,  26 

Homooblastic  texture,  200 

Koodoos,  103 

Hornfels,  125 
banded,  63 

Hornschiefer,  126 

Hutton,  66 

Hyaline  rocks,  6 

Hyalopilitic  texture,  199 

Hybrid  rocks,  135 

Hydrochemical  metamorphism,  164 

Hydrochloric  acid  from  volcanic  erup- 
tions, 84 

Hydrosphere,  6 

Hypidiomorphic-granular  texture,  197 

Hypocrystalline-porphyritic  texture, 
198 

Hysterocrystallization,  37 

Hysterogenic  schlieren,  44 


Jekaterinburg,  Urals,  39 
Jointing,  86,  184 

columnar,  186 
Juvenile  waters,  18 

K 

Kant,  14 

Kaolin  formation,  76 

Kaolinization,  149 

Karst  topography,  75 

Katamorphism,  78 

Kata-rocks,  174 

Katogenic  metamorphism,  164 

Kaulquappenquarze,  168 

Kidney  marble,  195 

Kilauea,  Hawaii,  27,  44 

Klingenberg,  81 

Klosterberg,  27 

Kneaded  texture,  171,  204 

Knoten  texture,  199 

Knotenglimmerschiefer,  126 

Knotenschiefer,  124,  126 


Iceland,  42,  54 
Idioblastic  texture,  200 
Idiomorphic,  197 
Igneous  rocks,  7 

age,  27 

Implication  texture,  198 
Included  constituents,  32 
Inclusions,  204 

endogenic,  204 

exogenic,  205 

metamorphism  of,  138 

primeval,  52 
Indirect  bedding,  101 
Injected-schist,  48 
Injection  rocks,  61 

-schist,  48 

-schlieren,  44 
Insolation,  74 
Intersertal  texture,  199 
Intratelluric  phenocrysts,  17,  42,  198 
Intrusive  rocks,  17,  24 
Isar,  58 
Isle  of  Skye,  47 


Laccoliths,  24 
Laminated  rocks,  191 
Lamprophyres,  49 
Lapilli,  25 
Laplace,  14 

Large-grained  rocks,  190 
Latent  plasticity,  165 
Laterite,  78,  83 
Lava  sheets,  26 

streams,  26 

Laws   of   association   for   contact-meta- 
morphism,  137 

of  minerals  in  igneous  rocks,  33 
Leidenfrost  phenomenon,  15 
Lepidoblastic  texture,  200 
Leucocratic,  48 
Liquation,  39,  65 
Literal  deposits,  100 
Loess,  98 
Low  Tauern,  59 
Luxullianite,  146 
Lydite,  138 


INDEX 


211 


M 


Magma,  18 

a  complex  solution,  22 
basins,  19 
explosiveness  of,  22 
physical  character  of,  21 
physico-chemical  laws  of,  36 
Maine,  52 
Marecanite,  187 
Marine  sediments,  99 
Marls,  97 

Martinique,  16,  21,  22,  27 
Massive  rocks,  7 
Mechanical  sediments,  95 

textures,  202 
Mediosilicic,  33,  35 
Medium-grained  rocks,  190 
Melanocratic,  48 
Meso-rocks,  174 
Mesostasis,  197 
Metagneisses,  163 
Metamorphism,  anogenic,  164 

dislocation,  164 

dynamo-,  164,  169 

hydrochemical,  164 

Neptunian,  164 

Plutonic,  164 
Meta-rocks,  173 
Metasomatic,  155 

replacement  of  carbonate  rocks,  155 
Miarolitic  rocks,  191 
Michel-Levy's  formulae,  68 
Microgranitic,  198 
Microlitic  texture,  199 
Micropegmatitic  texture,  198 
Migmatites,    135 
Mineral  laws  of  association,  33 
Mineral-dikes,  146 
Mineralizers,  18,  22,  164 

action  of,  41 

reduce  viscosity,  23 
Minerals,  accessory,  31,  32 

chief,  31 

essential,  31,  32 

included,  32 

primary,  32 

secondary,  32 

substitute,  31,  32 

unessential,  31 
Mixed  rocks,  5,  135 
Mixed-type,  67 


Mont  Blanc  fan  structure,  61 
Monzonitic  texture,  197 
Mortar  texture,  203 
'Mosaic  texture,  200 
Murbruk  texture,  203 
Mylonites,  170,  204 

N 

Nebulite,  65 

Xematoblastic  texture,  200 
Neptunian  metamorphism,  164 
Neubildungen,  76,  83 


Oberpfalz,  62 
Oblique  parting,  187 
Odenwald,  46,  64,  86 
Oolitic  texture,  202 
Ophitic  texture,  198,  200 
Ore-veins,  146 
Organic  weathering,  74,  84 
Organogenic  sediments,  80,  95 
Origin  of  border  zone  of  Alps,  135 
Orthogneisses,  163 
Orthophyric  texture,  199 
Ortho-rocks,  173 
Osann's  classification,  68 
diagrams,  72 


Palaopicrite,  28 

Palimpsest  texture,  118,  196,  200 
Panidiomorphic  texture,  198 
Panzerdecke,  19 
Paper-porphyry,  185 
Paragenesis  of  contact-rocks,  136 
Paragneisses,  163 
Para-rocks,  173 
Parting,  86,  184 

oblique,  187 

parallelopipedal,  188 

perlitic,  187 

spheroidal,  187 
Passau,  65 
Pegmatite,  formation  of,  142 

minerals  of,  143 
Pegmatitic  texture,  193 
Pelites,  96 
Penkatite,  130 


212 


INDEX 


Perlitic  parting,  187 
Persilicic,  33,  35 

Petrographic     character     and     geologic 
age,  13 

province,  19,  53 

Petroleum,  formation  of,  110,  114 
Petrology,  definition,  1 
Phanerites,  190 
Phenocrysts,  17 
Physical  weathering,  74 
Physico-chemical  laws  of  the  magma,  36 
Phytogenic  deposits,  107 
Piezo-contact-metamorphism,   134,   177 
Piezocrystallization,  55 
Pilotaxitic  texture,  199 
Pisolite,  formation  of,  104 
Pitchstone  contains  water,  18 
Plankton,  106 
Plasticity,  latent,  165 
Platy-parting,  185 

of  phonolite,  87 
Plutonic  metamorphism,  164 

rocks,  17 

Pneumatohydatogenic  processes,  139 
Pneumatolitic-metamorphism,  66 
Pneumatolitic  processes,  139 
Poikilitic  texture,  198 
Poikiloblastic  texture,  200 
Pompeii,  21 
Porcellanite,  137 
Porodine  deposits,  6 
Porous  rocks,  191 
Porphyritic  rocks,  17 

texture,  193,  200 
Porphyroblastic  texture,  200 
Porphyroblasts,  176 
Porphyroclastic  texture,  201 
Post-volcanic  processes,  139 
Pre-Cambrian  South  African  sediments, 

12 

Predazzite,  130 
Primary  minerals,  32 

rocks,  95 

Primeval  inclusions,  52 
Propylite,  148 
Propylitization,  148 
Proterobase^S 
Protoclaatic  texture,  203 
Protogine,  135,  172 
Psammites,  96 
Psephites,  96 
Pseudoporphyritic  texture,  199 


Putzen,  44 
Pyriboles,  32 


Q 


Quellkuppen,  26,  44 
Quiet  eruptions,  27 


II 


Random  texture,  191 

Recent  sediments,  114 

Regional  metamorphism,  156,  162 

Relation  of  folding  to  intrusion  in  Alps, 

59 

Replacement,  73,  140 
Resorption  37 

of  phenocrysts,  17 

-schlieren,  44 

Richtungslose,  texture,  46 
Riecke's  principle,  56,  170 
Ries,  Bavaria,  44] 
Riesenberg,  Bohemia,  63 
Rift,  87,  184 
Ripple-marks,  98 
Roches  moutonnees,  91 
Rock,  average,  34 

-sculpture,  85 

-seas,  87 

-streams,  87 

weathering,  73 
Rocks,  acid,  33 

abyssal,  17 

adiagnostic,  190 

anogenic,  7 

aphanitic,  190 

basic,  33 

bedded,  7 

catogenic,  7 

chemical  composition  of,  34 

clastic,  6 

compound,  5 

cryptogenic,  7 

cryptomerous,  190 

crystalline,  5 

deep-seated,  17,  24 

definition  of,  5 

effusive,  17 
•  extrusive,  17 

fragmental,  6 

fritted,  137 

glassy,  6 


INDEX 


213 


Rocks,  granular,  17 

hyaline,  6 

hybrid,  135 

igneous,  7 

intrusive,  17 

massive,  7 

mixed,  5,  135 

plutonic,  17 

porphyritic,  17 

primary,  95 

secondary,  95 

sedimentary,  7 

simple,  5 

surface,  17 
Rosenbusch's  graphical  method,  71 

kern  theory,  55 
Rubble,  96 
Rutile  needles,  96 


s 


Salt,  formation  of,  104 

-pans,  80 

-eelites,  99 
Sand,  volcanic,  25 
Sandstones,  96 
Satellite  dikes,  48 
Saussurite  hornfels  minerals,  136 
Saussuritization,  151 
Scaradrapass,  56 
Schistes  feldspatises,  67,  119 
Schistose  rocks,  191 
Schists,  crystalline,  7 
Schlieren,  39,  44 

constitution-,  44 

resorption-,  44 

injection-,  44 

differentiation-,  44 

hysterogenic-,  44 
Schlossberg,  Bohemia,  88 
Scoriaceous  rocks,  191 
Secondary  minerals,  32 

rocks,  95 

Secretions,  204,  205 
Sedimentary  rocks,  7 
Sediments,  seolian,  95,  97 

alluvial,  95,  99 

chemical,  95,  103 

composition  of,  95 

fluviatile,  99 

fossil,  114 

glacial,  95,  102 


Sediments,  marine,  99 

mechanical,  95 

organogenic,  80,  95,  106 

recent,  114 
Septaria,  205 
Sequence  of  crystallization  in  granite,  197 

of  minerals,  37,  38 
Sericitization,  152 
Serpentinization,  152 

by  contact-metamorphism,  132 
Sheets,  24,  26 
Sieve  texture,  118,  199 
Silicification,  154 
Simple  rocks,  5 
Skarn,  129 

Slate-hornfels  minerals,  136 
Soil-zeolites,  77 

South  Africa  unfossiliferous  sediments,  12 
Specific  gravity  of  earth,  8 

of  earth's  crust,  8 
Spheroidal  parting,  187 

texture,  193 

weathering  of  basaltic  columns,  88 

of  diabase,  87 

Spherulitic  groundmass,  198 
Spilosites,  138 
Spring's  experiments,  169 
Stalactites,  formation  of,  104 
Steinheimer  Basin,  27 
Stellvertreter,  31 
Stigmaria,  109 
Stink-stone,  130 
Stocks,  24 
Strato-volcanoes,  26 
Streams,  lava,  26 
Stretched  sandstone,  168 
Stiibel's  theory,  19,  178 
Subsilicic,  33,  35 
Substitute  minerals,  31 
Surface  rocks,  17 
Sutured  texture,  200 
Swabian  Alb,  21,  44 


Tad-pole  quartz,  168 
Talc  formation,  153 
Temperature  at  earth's  center,  8 

gradient,  8 

within  the  earth,  14    . 
Terra  rossa,  75 
Texture,  andesitic,  199 


214 


INDEX 


Texture,  aplitic,  198 

blastogranitic,  200 

blastophitic,  201 

blastoporphyritic,  201 

blastopsammitic,  201 

blastopsephitic,  201 

cataclastic,  203 

crystalloblastic,  200 

cyclopic,  200 

diablastic,  200 

fluctuation,  191 

fluidal,  191 

gabbroic,  198 

garben,  199 

granitic,  39,  197 

granoblastic,  176,  200 

granular,  197 

granulitic,  39,  198 

haphazard,  46,  191 

helizitic,  118,  200 

hole-crystalline,  197 

homooblastic,  200 

hyalopilitic,  199 

hypidiomorphic-granular,  197 

idioblastic,  200 

implication,  198 

intersertal,  199 

kneaded,  171,  204 
knoten,  199 
lepidoblastic,  200 
mechanical,  202 
microlitic,  199 
micropegmatitic,  198 
monzonitic,  197 
mortar,  203 
mosaic,  200 
murbruk,  203 
nematoblastic,  200 
oolitic,  202 
ophitic,  198,  200 
orthophyric,  199 
palimpsest,  118,  196,  200 
panidiomorphic,  198 
pegmatitic,  193 
pilotaxitic,  199 
poikilitic,  198 
poikiloblastic,  200 
porphyritic,  193,  198,  200 
porphyroblastic,  176,  200 
porphyroclastic,  201 
protoclastic,  203 
pseudoporphyritic,  199 


Texture,  random,  191 

richtungslose,  46 

sieve,  118,  199 

spheroidal,  193 

sutured,  200 

trachytic,  198 

xenoblastic,  200 
Textures  of  contact-rocks,  199 

of  crystalline  schists,  199 

of  igneous  rocks,  196 

of  sedimentary  rocks,  201 
Thermal  processes,  139 
Thiiringen,  48 
Trachytic  texture,  198 
Tuff,  26 

Two  generations  of  crystals,  17 
Type-mixing,  66 

W 

Washakie  Basin,  Wyoming,  90 
Waters,  juvenile,  18 

magmatic,  18 

vadose,  18 

Weathered  residues,  80 
Weathering,  73 

chemical,  74 

organic,  74,  84 

physical,  74 

solutions,  79. 
Webern,  Odenwald,  46 
Welded  dikes,  52 
Wener  Sea,  Sweden,  42 
Werner's  theory  of  vulcanism,  14 
Wiesalpe,  Dachstein,  75 
Wiesbaden  limestone,  7 

X 

Xenoblastic  texture,  200 
Xenomorphic,  198 


Yellowstone  Park,  42 
Younger  crystalline  schists,  158 


Zeolitization,  154 
Zillertal,  133 
Zoogenic  limestones,  106 
Zwitter,  146 


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UNIVERSITY  OF  CALIFORNIA  LIBRARY 


